adding jet as a generator

_examples/scatter/main.go

@@ -17,7 +17,7 @@ func drawChart(res http.ResponseWriter, req *http.Request) {
 					Show:             true,
 					StrokeWidth:      chart.Disabled,
 					DotWidth:         5,
-					DotColorProvider: chart.Viridis,
+					DotColorProvider: chart.Jet,
 				},
 				XValues: chart.Sequence.Random(128, 1024),
 				YValues: chart.Sequence.Random(128, 1024),

drawing/color.go

@@ -57,6 +57,11 @@ func ColorFromAlphaMixedRGBA(r, g, b, a uint32) Color {
 	return c
 }
 
+// ColorChannelFromFloat returns a normalized byte from a given float value.
+func ColorChannelFromFloat(v float64) uint8 {
+	return uint8(v * 255)
+}
+
 // Color is our internal color type because color.Color is bullshit.
 type Color struct {
 	R, G, B, A uint8

jet.go

@@ -0,0 +1,37 @@
+package chart
+
+import "github.com/wcharczuk/go-chart/drawing"
+
+// Jet is a color map provider based on matlab's jet color map.
+func Jet(xr, yr Range, x, y float64) drawing.Color {
+	return getJetColour(y, yr.GetMin(), yr.GetMax())
+}
+
+func getJetColour(v, vmin, vmax float64) drawing.Color {
+	c := drawing.Color{R: 0xff, G: 0xff, B: 0xff, A: 0xff} // white
+	var dv float64
+
+	if v < vmin {
+		v = vmin
+	}
+	if v > vmax {
+		v = vmax
+	}
+	dv = vmax - vmin
+
+	if v < (vmin + 0.25*dv) {
+		c.R = 0
+		c.G = drawing.ColorChannelFromFloat(4 * (v - vmin) / dv)
+	} else if v < (vmin + 0.5*dv) {
+		c.R = 0
+		c.B = drawing.ColorChannelFromFloat(1 + 4*(vmin+0.25*dv-v)/dv)
+	} else if v < (vmin + 0.75*dv) {
+		c.R = drawing.ColorChannelFromFloat(4 * (v - vmin - 0.5*dv) / dv)
+		c.B = 0
+	} else {
+		c.G = drawing.ColorChannelFromFloat(1 + 4*(vmin+0.75*dv-v)/dv)
+		c.B = 0
+	}
+
+	return c
+}