func (p *smoothDiverging) At(v float64) (color.Color, error) {
	if err := checkRange(p.min, p.max, v); err != nil {
		return nil, err
	}
	convergePoint := (p.convergePoint - p.min) / (p.max - p.min)
	scalar := (v - p.min) / (p.max - p.min)
	o := p.interpolateMSHDiverging(scalar, convergePoint).cieLAB().cieXYZ().rgb().sRGBA(p.alpha)
	if !inUnitRange(o.R) || !inUnitRange(o.G) || !inUnitRange(o.B) || !inUnitRange(o.A) {
		return nil, fmt.Errorf("moreland: invalid color r:%g, g:%g, b:%g, a:%g", o.R, o.G, o.B, o.A)
	}
	return o, nil
}
At implements the palette.ColorMap interface.
package moreland
import (
	"fmt"
	"image/color"
	"math"
	"gonum.org/v1/plot/palette"
)
type smoothDiverging struct {
	start, end msh
	convergeM float64
	alpha float64
	min, max float64
	convergePoint float64
}
func NewSmoothDiverging(start, end color.Color, convergeM float64) palette.DivergingColorMap {
	return newSmoothDiverging(colorToMSH(start), colorToMSH(end), convergeM)
}
func newSmoothDiverging(start, end msh, convergeM float64) palette.DivergingColorMap {
	return &smoothDiverging{
		start:         start,
		end:           end,
		convergeM:     convergeM,
		convergePoint: math.NaN(),
		alpha:         1,
	}
}