func Shape(i int) draw.GlyphDrawer {
	n := len(DefaultGlyphShapes)
	if i < 0 {
		return DefaultGlyphShapes[i%n+n]
	}
	return DefaultGlyphShapes[i%n]
}
Shape returns the ith default glyph shape,
wrapping if i is less than zero or greater
than the max number of GlyphDrawers
in the DefaultGlyphShapes slice.
package plotutil  
import (
	"image/color"
	"gonum.org/v1/plot/vg"
	"gonum.org/v1/plot/vg/draw"
)
var DefaultColors = SoftColors
var DarkColors = []color.Color{
	rgb(238, 46, 47),
	rgb(0, 140, 72),
	rgb(24, 90, 169),
	rgb(244, 125, 35),
	rgb(102, 44, 145),
	rgb(162, 29, 33),
	rgb(180, 56, 148),
}
var SoftColors = []color.Color{
	rgb(241, 90, 96),
	rgb(122, 195, 106),
	rgb(90, 155, 212),
	rgb(250, 167, 91),
	rgb(158, 103, 171),
	rgb(206, 112, 88),
	rgb(215, 127, 180),
}
func rgb(r, g, b uint8) color.RGBA {
	return color.RGBA{r, g, b, 255}
}
func Color(i int) color.Color {
	n := len(DefaultColors)
	if i < 0 {
		return DefaultColors[i%n+n]
	}
	return DefaultColors[i%n]
}
var DefaultGlyphShapes = []draw.GlyphDrawer{
	draw.RingGlyph{},
	draw.SquareGlyph{},
	draw.TriangleGlyph{},
	draw.CrossGlyph{},
	draw.PlusGlyph{},
	draw.CircleGlyph{},
	draw.BoxGlyph{},
	draw.PyramidGlyph{},
}