//
//  LineChartRenderer.swift
//  Charts
//
//  Copyright 2015 Daniel Cohen Gindi & Philipp Jahoda
//  A port of MPAndroidChart for iOS
//  Licensed under Apache License 2.0
//
//  https://github.com/danielgindi/Charts
//

import Foundation
import CoreGraphics

open class LineChartRenderer: LineRadarRenderer
{
    // TODO: Currently, this nesting isn't necessary for LineCharts. However, it will make it much easier to add a custom rotor
    // that navigates between datasets.
    // NOTE: Unlike the other renderers, LineChartRenderer populates accessibleChartElements in drawCircles due to the nature of its drawing options.
    /// A nested array of elements ordered logically (i.e not in visual/drawing order) for use with VoiceOver.
    private lazy var accessibilityOrderedElements: [[NSUIAccessibilityElement]] = accessibilityCreateEmptyOrderedElements()

    @objc open weak var dataProvider: LineChartDataProvider?
    
    @objc public init(dataProvider: LineChartDataProvider, animator: Animator, viewPortHandler: ViewPortHandler)
    {
        super.init(animator: animator, viewPortHandler: viewPortHandler)
        
        self.dataProvider = dataProvider
    }
    
    open override func drawData(context: CGContext)
    {
        guard let lineData = dataProvider?.lineData else { return }
        
        for i in 0 ..< lineData.dataSetCount
        {
            guard let set = lineData.getDataSetByIndex(i) else { continue }
            
            if set.isVisible
            {
                if !(set is ILineChartDataSet)
                {
                    fatalError("Datasets for LineChartRenderer must conform to ILineChartDataSet")
                }
                
                drawDataSet(context: context, dataSet: set as! ILineChartDataSet)
            }
        }
    }
    
    @objc open func drawDataSet(context: CGContext, dataSet: ILineChartDataSet)
    {
        if dataSet.entryCount < 1
        {
            return
        }
        
        context.saveGState()
        
        context.setLineWidth(dataSet.lineWidth)
        if dataSet.lineDashLengths != nil
        {
            context.setLineDash(phase: dataSet.lineDashPhase, lengths: dataSet.lineDashLengths!)
        }
        else
        {
            context.setLineDash(phase: 0.0, lengths: [])
        }
        
        context.setLineCap(dataSet.lineCapType)
        
        // if drawing cubic lines is enabled
        switch dataSet.mode
        {
        case .linear: fallthrough
        case .stepped:
            drawLinear(context: context, dataSet: dataSet)
            
        case .cubicBezier:
            drawCubicBezier(context: context, dataSet: dataSet)
            
        case .horizontalBezier:
            drawHorizontalBezier(context: context, dataSet: dataSet)
        }
        
        context.restoreGState()
    }
    
    @objc open func drawCubicBezier(context: CGContext, dataSet: ILineChartDataSet)
    {
        guard let dataProvider = dataProvider else { return }
        
        let trans = dataProvider.getTransformer(forAxis: dataSet.axisDependency)
        
        let phaseY = animator.phaseY
        
        _xBounds.set(chart: dataProvider, dataSet: dataSet, animator: animator)
        
        // get the color that is specified for this position from the DataSet
        let drawingColor = dataSet.colors.first!
        
        let intensity = dataSet.cubicIntensity
        
        // the path for the cubic-spline
        let cubicPath = CGMutablePath()
        
        let valueToPixelMatrix = trans.valueToPixelMatrix
        
        if _xBounds.range >= 1
        {
            var prevDx: CGFloat = 0.0
            var prevDy: CGFloat = 0.0
            var curDx: CGFloat = 0.0
            var curDy: CGFloat = 0.0
            
            // Take an extra point from the left, and an extra from the right.
            // That's because we need 4 points for a cubic bezier (cubic=4), otherwise we get lines moving and doing weird stuff on the edges of the chart.
            // So in the starting `prev` and `cur`, go -2, -1
            
            let firstIndex = _xBounds.min + 1
            
            var prevPrev: ChartDataEntry! = nil
            var prev: ChartDataEntry! = dataSet.entryForIndex(max(firstIndex - 2, 0))
            var cur: ChartDataEntry! = dataSet.entryForIndex(max(firstIndex - 1, 0))
            var next: ChartDataEntry! = cur
            var nextIndex: Int = -1
            
            if cur == nil { return }
            
            // let the spline start
            cubicPath.move(to: CGPoint(x: CGFloat(cur.x), y: CGFloat(cur.y * phaseY)), transform: valueToPixelMatrix)
            
            for j in _xBounds.dropFirst()  // same as firstIndex
            {
                prevPrev = prev
                prev = cur
                cur = nextIndex == j ? next : dataSet.entryForIndex(j)
                
                nextIndex = j + 1 < dataSet.entryCount ? j + 1 : j
                next = dataSet.entryForIndex(nextIndex)
                
                if next == nil { break }
                
                prevDx = CGFloat(cur.x - prevPrev.x) * intensity
                prevDy = CGFloat(cur.y - prevPrev.y) * intensity
                curDx = CGFloat(next.x - prev.x) * intensity
                curDy = CGFloat(next.y - prev.y) * intensity
                
                cubicPath.addCurve(
                    to: CGPoint(
                        x: CGFloat(cur.x),
                        y: CGFloat(cur.y) * CGFloat(phaseY)),
                    control1: CGPoint(
                        x: CGFloat(prev.x) + prevDx,
                        y: (CGFloat(prev.y) + prevDy) * CGFloat(phaseY)),
                    control2: CGPoint(
                        x: CGFloat(cur.x) - curDx,
                        y: (CGFloat(cur.y) - curDy) * CGFloat(phaseY)),
                    transform: valueToPixelMatrix)
            }
        }
        
        context.saveGState()
        
        if dataSet.isDrawFilledEnabled
        {
            // Copy this path because we make changes to it
            let fillPath = cubicPath.mutableCopy()
            
            drawCubicFill(context: context, dataSet: dataSet, spline: fillPath!, matrix: valueToPixelMatrix, bounds: _xBounds)
        }
        
        context.beginPath()
        context.addPath(cubicPath)
        context.setStrokeColor(drawingColor.cgColor)
        context.strokePath()
        
        context.restoreGState()
    }
    
    @objc open func drawHorizontalBezier(context: CGContext, dataSet: ILineChartDataSet)
    {
        guard let dataProvider = dataProvider else { return }
        
        let trans = dataProvider.getTransformer(forAxis: dataSet.axisDependency)
        
        let phaseY = animator.phaseY
        
        _xBounds.set(chart: dataProvider, dataSet: dataSet, animator: animator)
        
        // get the color that is specified for this position from the DataSet
        let drawingColor = dataSet.colors.first!
        
        // the path for the cubic-spline
        let cubicPath = CGMutablePath()
        
        let valueToPixelMatrix = trans.valueToPixelMatrix
        
        if _xBounds.range >= 1
        {
            var prev: ChartDataEntry! = dataSet.entryForIndex(_xBounds.min)
            var cur: ChartDataEntry! = prev
            
            if cur == nil { return }
            
            // let the spline start
            cubicPath.move(to: CGPoint(x: CGFloat(cur.x), y: CGFloat(cur.y * phaseY)), transform: valueToPixelMatrix)
            
            for j in _xBounds.dropFirst()
            {
                prev = cur
                cur = dataSet.entryForIndex(j)
                
                let cpx = CGFloat(prev.x + (cur.x - prev.x) / 2.0)
                
                cubicPath.addCurve(
                    to: CGPoint(
                        x: CGFloat(cur.x),
                        y: CGFloat(cur.y * phaseY)),
                    control1: CGPoint(
                        x: cpx,
                        y: CGFloat(prev.y * phaseY)),
                    control2: CGPoint(
                        x: cpx,
                        y: CGFloat(cur.y * phaseY)),
                    transform: valueToPixelMatrix)
            }
        }
        
        context.saveGState()
        
        if dataSet.isDrawFilledEnabled
        {
            // Copy this path because we make changes to it
            let fillPath = cubicPath.mutableCopy()
            
            drawCubicFill(context: context, dataSet: dataSet, spline: fillPath!, matrix: valueToPixelMatrix, bounds: _xBounds)
        }
        
        context.beginPath()
        context.addPath(cubicPath)
        context.setStrokeColor(drawingColor.cgColor)
        context.strokePath()
        
        context.restoreGState()
    }
    
    open func drawCubicFill(
        context: CGContext,
                dataSet: ILineChartDataSet,
                spline: CGMutablePath,
                matrix: CGAffineTransform,
                bounds: XBounds)
    {
        guard
            let dataProvider = dataProvider
            else { return }
        
        if bounds.range <= 0
        {
            return
        }
        
        let fillMin = dataSet.fillFormatter?.getFillLinePosition(dataSet: dataSet, dataProvider: dataProvider) ?? 0.0

        var pt1 = CGPoint(x: CGFloat(dataSet.entryForIndex(bounds.min + bounds.range)?.x ?? 0.0), y: fillMin)
        var pt2 = CGPoint(x: CGFloat(dataSet.entryForIndex(bounds.min)?.x ?? 0.0), y: fillMin)
        pt1 = pt1.applying(matrix)
        pt2 = pt2.applying(matrix)
        
        spline.addLine(to: pt1)
        spline.addLine(to: pt2)
        spline.closeSubpath()
        
        if dataSet.fill != nil
        {
            drawFilledPath(context: context, path: spline, fill: dataSet.fill!, fillAlpha: dataSet.fillAlpha)
        }
        else
        {
            drawFilledPath(context: context, path: spline, fillColor: dataSet.fillColor, fillAlpha: dataSet.fillAlpha)
        }
    }
    
    private var _lineSegments = [CGPoint](repeating: CGPoint(), count: 2)
    
    @objc open func drawLinear(context: CGContext, dataSet: ILineChartDataSet)
    {
        guard let dataProvider = dataProvider else { return }
        
        let trans = dataProvider.getTransformer(forAxis: dataSet.axisDependency)
        
        let valueToPixelMatrix = trans.valueToPixelMatrix
        
        let entryCount = dataSet.entryCount
        let isDrawSteppedEnabled = dataSet.mode == .stepped
        let pointsPerEntryPair = isDrawSteppedEnabled ? 4 : 2
        
        let phaseY = animator.phaseY
        
        _xBounds.set(chart: dataProvider, dataSet: dataSet, animator: animator)
        
        // if drawing filled is enabled
        if dataSet.isDrawFilledEnabled && entryCount > 0
        {
            drawLinearFill(context: context, dataSet: dataSet, trans: trans, bounds: _xBounds)
        }
        
        context.saveGState()

            if _lineSegments.count != pointsPerEntryPair
            {
                // Allocate once in correct size
                _lineSegments = [CGPoint](repeating: CGPoint(), count: pointsPerEntryPair)
            }

        for j in _xBounds.dropLast()
        {
            var e: ChartDataEntry! = dataSet.entryForIndex(j)
            
            if e == nil { continue }
            
            _lineSegments[0].x = CGFloat(e.x)
            _lineSegments[0].y = CGFloat(e.y * phaseY)
            
            if j < _xBounds.max
            {
                // TODO: remove the check.
                // With the new XBounds iterator, j is always smaller than _xBounds.max
                // Keeping this check for a while, if xBounds have no further breaking changes, it should be safe to remove the check
                e = dataSet.entryForIndex(j + 1)
                
                if e == nil { break }
                
                if isDrawSteppedEnabled
                {
                    _lineSegments[1] = CGPoint(x: CGFloat(e.x), y: _lineSegments[0].y)
                    _lineSegments[2] = _lineSegments[1]
                    _lineSegments[3] = CGPoint(x: CGFloat(e.x), y: CGFloat(e.y * phaseY))
                }
                else
                {
                    _lineSegments[1] = CGPoint(x: CGFloat(e.x), y: CGFloat(e.y * phaseY))
                }
            }
            else
            {
                _lineSegments[1] = _lineSegments[0]
            }

            for i in 0..<_lineSegments.count
            {
                _lineSegments[i] = _lineSegments[i].applying(valueToPixelMatrix)
            }
            
            if !viewPortHandler.isInBoundsRight(_lineSegments[0].x)
            {
                break
            }
            
            // Determine the start and end coordinates of the line, and make sure they differ.
            guard
                let firstCoordinate = _lineSegments.first,
                let lastCoordinate = _lineSegments.last,
                firstCoordinate != lastCoordinate else { continue }
            
            // make sure the lines don't do shitty things outside bounds
            if !viewPortHandler.isInBoundsLeft(lastCoordinate.x) ||
                !viewPortHandler.isInBoundsTop(max(firstCoordinate.y, lastCoordinate.y)) ||
                !viewPortHandler.isInBoundsBottom(min(firstCoordinate.y, lastCoordinate.y))
            {
                continue
            }
            
            // get the color that is set for this line-segment
            context.setStrokeColor(dataSet.color(atIndex: j).cgColor)
            context.strokeLineSegments(between: _lineSegments)
        }
        
        context.restoreGState()
    }
    
    open func drawLinearFill(context: CGContext, dataSet: ILineChartDataSet, trans: Transformer, bounds: XBounds)
    {
        guard let dataProvider = dataProvider else { return }
        
        let filled = generateFilledPath(
            dataSet: dataSet,
            fillMin: dataSet.fillFormatter?.getFillLinePosition(dataSet: dataSet, dataProvider: dataProvider) ?? 0.0,
            bounds: bounds,
            matrix: trans.valueToPixelMatrix)
        
        if dataSet.fill != nil
        {
            drawFilledPath(context: context, path: filled, fill: dataSet.fill!, fillAlpha: dataSet.fillAlpha)
        }
        else
        {
            drawFilledPath(context: context, path: filled, fillColor: dataSet.fillColor, fillAlpha: dataSet.fillAlpha)
        }
    }
    
    /// Generates the path that is used for filled drawing.
    private func generateFilledPath(dataSet: ILineChartDataSet, fillMin: CGFloat, bounds: XBounds, matrix: CGAffineTransform) -> CGPath
    {
        let phaseY = animator.phaseY
        let isDrawSteppedEnabled = dataSet.mode == .stepped
        let matrix = matrix
        
        var e: ChartDataEntry!
        
        let filled = CGMutablePath()
        
        e = dataSet.entryForIndex(bounds.min)
        if e != nil
        {
            filled.move(to: CGPoint(x: CGFloat(e.x), y: fillMin), transform: matrix)
            filled.addLine(to: CGPoint(x: CGFloat(e.x), y: CGFloat(e.y * phaseY)), transform: matrix)
        }
        
        // create a new path
        for x in stride(from: (bounds.min + 1), through: bounds.range + bounds.min, by: 1)
        {
            guard let e = dataSet.entryForIndex(x) else { continue }
            
            if isDrawSteppedEnabled
            {
                guard let ePrev = dataSet.entryForIndex(x-1) else { continue }
                filled.addLine(to: CGPoint(x: CGFloat(e.x), y: CGFloat(ePrev.y * phaseY)), transform: matrix)
            }
            
            filled.addLine(to: CGPoint(x: CGFloat(e.x), y: CGFloat(e.y * phaseY)), transform: matrix)
        }
        
        // close up
        e = dataSet.entryForIndex(bounds.range + bounds.min)
        if e != nil
        {
            filled.addLine(to: CGPoint(x: CGFloat(e.x), y: fillMin), transform: matrix)
        }
        filled.closeSubpath()
        
        return filled
    }
    
    open override func drawValues(context: CGContext)
    {
        guard
            let dataProvider = dataProvider,
            let lineData = dataProvider.lineData
            else { return }

        if isDrawingValuesAllowed(dataProvider: dataProvider)
        {
            let dataSets = lineData.dataSets
            
            let phaseY = animator.phaseY
            
            var pt = CGPoint()
            
            for i in 0 ..< dataSets.count
            {
                guard let
                    dataSet = dataSets[i] as? ILineChartDataSet,
                    shouldDrawValues(forDataSet: dataSet)
                    else { continue }
                
                let valueFont = dataSet.valueFont
                
                guard let formatter = dataSet.valueFormatter else { continue }
                
                let trans = dataProvider.getTransformer(forAxis: dataSet.axisDependency)
                let valueToPixelMatrix = trans.valueToPixelMatrix
                
                let iconsOffset = dataSet.iconsOffset
                
                // make sure the values do not interfear with the circles
                var valOffset = Int(dataSet.circleRadius * 1.75)
                
                if !dataSet.isDrawCirclesEnabled
                {
                    valOffset = valOffset / 2
                }
                
                _xBounds.set(chart: dataProvider, dataSet: dataSet, animator: animator)

                for j in _xBounds
                {
                    guard let e = dataSet.entryForIndex(j) else { break }
                    
                    pt.x = CGFloat(e.x)
                    pt.y = CGFloat(e.y * phaseY)
                    pt = pt.applying(valueToPixelMatrix)
                    
                    if (!viewPortHandler.isInBoundsRight(pt.x))
                    {
                        break
                    }
                    
                    if (!viewPortHandler.isInBoundsLeft(pt.x) || !viewPortHandler.isInBoundsY(pt.y))
                    {
                        continue
                    }
                    
                    if dataSet.isDrawValuesEnabled {
                        ChartUtils.drawText(
                            context: context,
                            text: formatter.stringForValue(
                                e.y,
                                entry: e,
                                dataSetIndex: i,
                                viewPortHandler: viewPortHandler),
                            point: CGPoint(
                                x: pt.x,
                                y: pt.y - CGFloat(valOffset) - valueFont.lineHeight),
                            align: .center,
                            attributes: [NSAttributedString.Key.font: valueFont, NSAttributedString.Key.foregroundColor: dataSet.valueTextColorAt(j)])
                    }
                    
                    if let icon = e.icon, dataSet.isDrawIconsEnabled
                    {
                        ChartUtils.drawImage(context: context,
                                             image: icon,
                                             x: pt.x + iconsOffset.x,
                                             y: pt.y + iconsOffset.y,
                                             size: icon.size)
                    }
                }
            }
        }
    }
    
    open override func drawExtras(context: CGContext)
    {
        drawCircles(context: context)
    }
    
    private func drawCircles(context: CGContext)
    {
        guard
            let dataProvider = dataProvider,
            let lineData = dataProvider.lineData
            else { return }
        
        let phaseY = animator.phaseY

        let dataSets = lineData.dataSets
        
        var pt = CGPoint()
        var rect = CGRect()
        
        // If we redraw the data, remove and repopulate accessible elements to update label values and frames
        accessibleChartElements.removeAll()
        accessibilityOrderedElements = accessibilityCreateEmptyOrderedElements()

        // Make the chart header the first element in the accessible elements array
        if let chart = dataProvider as? LineChartView {
            let element = createAccessibleHeader(usingChart: chart,
                                                 andData: lineData,
                                                 withDefaultDescription: "Line Chart")
            accessibleChartElements.append(element)
        }

        context.saveGState()

        for i in 0 ..< dataSets.count
        {
            guard let dataSet = lineData.getDataSetByIndex(i) as? ILineChartDataSet else { continue }
            
            if !dataSet.isVisible || !dataSet.isDrawCirclesEnabled || dataSet.entryCount == 0
            {
                continue
            }
            
            let trans = dataProvider.getTransformer(forAxis: dataSet.axisDependency)
            let valueToPixelMatrix = trans.valueToPixelMatrix
            
            _xBounds.set(chart: dataProvider, dataSet: dataSet, animator: animator)
            
            let circleRadius = dataSet.circleRadius
            let circleDiameter = circleRadius * 2.0
            let circleHoleRadius = dataSet.circleHoleRadius
            let circleHoleDiameter = circleHoleRadius * 2.0
            
            let drawCircleHole = dataSet.isDrawCircleHoleEnabled &&
                circleHoleRadius < circleRadius &&
                circleHoleRadius > 0.0
            let drawTransparentCircleHole = drawCircleHole &&
                (dataSet.circleHoleColor == nil ||
                    dataSet.circleHoleColor == NSUIColor.clear)
            
            for j in _xBounds
            {
                guard let e = dataSet.entryForIndex(j) else { break }

                pt.x = CGFloat(e.x)
                pt.y = CGFloat(e.y * phaseY)
                pt = pt.applying(valueToPixelMatrix)
                
                if (!viewPortHandler.isInBoundsRight(pt.x))
                {
                    break
                }
                
                // make sure the circles don't do shitty things outside bounds
                if (!viewPortHandler.isInBoundsLeft(pt.x) || !viewPortHandler.isInBoundsY(pt.y))
                {
                    continue
                }
                
                
                // Skip Circles and Accessibility if not enabled,
                // reduces CPU significantly if not needed
                if !dataSet.isDrawCirclesEnabled
                {
                    continue
                }
                
                // Accessibility element geometry
                let scaleFactor: CGFloat = 3
                let accessibilityRect = CGRect(x: pt.x - (scaleFactor * circleRadius),
                                               y: pt.y - (scaleFactor * circleRadius),
                                               width: scaleFactor * circleDiameter,
                                               height: scaleFactor * circleDiameter)
                // Create and append the corresponding accessibility element to accessibilityOrderedElements
                if let chart = dataProvider as? LineChartView
                {
                    let element = createAccessibleElement(withIndex: j,
                                                          container: chart,
                                                          dataSet: dataSet,
                                                          dataSetIndex: i)
                    { (element) in
                        element.accessibilityFrame = accessibilityRect
                    }

                    accessibilityOrderedElements[i].append(element)
                }

                context.setFillColor(dataSet.getCircleColor(atIndex: j)!.cgColor)

                rect.origin.x = pt.x - circleRadius
                rect.origin.y = pt.y - circleRadius
                rect.size.width = circleDiameter
                rect.size.height = circleDiameter

                if drawTransparentCircleHole
                {
                    // Begin path for circle with hole
                    context.beginPath()
                    context.addEllipse(in: rect)
                    
                    // Cut hole in path
                    rect.origin.x = pt.x - circleHoleRadius
                    rect.origin.y = pt.y - circleHoleRadius
                    rect.size.width = circleHoleDiameter
                    rect.size.height = circleHoleDiameter
                    context.addEllipse(in: rect)
                    
                    // Fill in-between
                    context.fillPath(using: .evenOdd)
                }
                else
                {
                    context.fillEllipse(in: rect)
                    
                    if drawCircleHole
                    {
                        context.setFillColor(dataSet.circleHoleColor!.cgColor)
                     
                        // The hole rect
                        rect.origin.x = pt.x - circleHoleRadius
                        rect.origin.y = pt.y - circleHoleRadius
                        rect.size.width = circleHoleDiameter
                        rect.size.height = circleHoleDiameter
                        
                        context.fillEllipse(in: rect)
                    }
                }
            }
        }
        
        context.restoreGState()

        // Merge nested ordered arrays into the single accessibleChartElements.
        accessibleChartElements.append(contentsOf: accessibilityOrderedElements.flatMap { $0 } )
        accessibilityPostLayoutChangedNotification()
    }
    
    open override func drawHighlighted(context: CGContext, indices: [Highlight])
    {
        guard
            let dataProvider = dataProvider,
            let lineData = dataProvider.lineData
            else { return }
        
        let chartXMax = dataProvider.chartXMax
        
        context.saveGState()
        
        for high in indices
        {
            guard let set = lineData.getDataSetByIndex(high.dataSetIndex) as? ILineChartDataSet
                , set.isHighlightEnabled
                else { continue }
            
            guard let e = set.entryForXValue(high.x, closestToY: high.y) else { continue }
            
            if !isInBoundsX(entry: e, dataSet: set)
            {
                continue
            }
        
            context.setStrokeColor(set.highlightColor.cgColor)
            context.setLineWidth(set.highlightLineWidth)
            if set.highlightLineDashLengths != nil
            {
                context.setLineDash(phase: set.highlightLineDashPhase, lengths: set.highlightLineDashLengths!)
            }
            else
            {
                context.setLineDash(phase: 0.0, lengths: [])
            }
            
            let x = e.x // get the x-position
            let y = e.y * Double(animator.phaseY)
            
            if x > chartXMax * animator.phaseX
            {
                continue
            }
            
            let trans = dataProvider.getTransformer(forAxis: set.axisDependency)
            
            let pt = trans.pixelForValues(x: x, y: y)
            
            high.setDraw(pt: pt)
            
            // draw the lines
            drawHighlightLines(context: context, point: pt, set: set)
        }
        
        context.restoreGState()
    }

    /// Creates a nested array of empty subarrays each of which will be populated with NSUIAccessibilityElements.
    /// This is marked internal to support HorizontalBarChartRenderer as well.
    private func accessibilityCreateEmptyOrderedElements() -> [[NSUIAccessibilityElement]]
    {
        guard let chart = dataProvider as? LineChartView else { return [] }

        let dataSetCount = chart.lineData?.dataSetCount ?? 0

        return Array(repeating: [NSUIAccessibilityElement](),
                     count: dataSetCount)
    }

    /// Creates an NSUIAccessibleElement representing the smallest meaningful bar of the chart
    /// i.e. in case of a stacked chart, this returns each stack, not the combined bar.
    /// Note that it is marked internal to support subclass modification in the HorizontalBarChart.
    private func createAccessibleElement(withIndex idx: Int,
                                          container: LineChartView,
                                          dataSet: ILineChartDataSet,
                                          dataSetIndex: Int,
                                          modifier: (NSUIAccessibilityElement) -> ()) -> NSUIAccessibilityElement
    {
        let element = NSUIAccessibilityElement(accessibilityContainer: container)
        let xAxis = container.xAxis

        guard let e = dataSet.entryForIndex(idx) else { return element }
        guard let dataProvider = dataProvider else { return element }

        // NOTE: The formatter can cause issues when the x-axis labels are consecutive ints.
        // i.e. due to the Double conversion, if there are more than one data set that are grouped,
        // there is the possibility of some labels being rounded up. A floor() might fix this, but seems to be a brute force solution.
        let label = xAxis.valueFormatter?.stringForValue(e.x, axis: xAxis) ?? "\(e.x)"

        let elementValueText = dataSet.valueFormatter?.stringForValue(e.y,
                                                                      entry: e,
                                                                      dataSetIndex: dataSetIndex,
                                                                      viewPortHandler: viewPortHandler) ?? "\(e.y)"

        let dataSetCount = dataProvider.lineData?.dataSetCount ?? -1
        let doesContainMultipleDataSets = dataSetCount > 1

        element.accessibilityLabel = "\(doesContainMultipleDataSets ? (dataSet.label ?? "")  + ", " : "") \(label): \(elementValueText)"

        modifier(element)

        return element
    }
}
