Add thumbnailer for palette.Palette

plotter/heat_test.go

@@ -5,6 +5,7 @@
 package plotter
 
 import (
+	"fmt"
 	"log"
 	"testing"
 
@@ -48,7 +49,8 @@ func ExampleHeatMap() {
 			5, 6, 7, 8,
 			9, 10, 11, 12,
 		})}
-	h := NewHeatMap(m, palette.Heat(12, 1))
+	plte := palette.Heat(12, 1)
+	h := NewHeatMap(m, plte)
 
 	p, err := plot.New()
 	if err != nil {
@@ -58,6 +60,16 @@ func ExampleHeatMap() {
 
 	p.Add(h)
 
+	// Create a legend.
+	rng := h.Max - h.Min
+	thumbs := PaletteThumbnailers(plte)
+	for i := len(thumbs) - 1; i >= 0; i-- {
+		t := thumbs[i]
+		fmin := float64(i) / float64(len(thumbs))
+		fmax := float64(i+1) / float64(len(thumbs))
+		p.Legend.Add(fmt.Sprintf("%.2g - %.2g", h.Min+fmin*rng, h.Min+fmax*rng), t)
+	}
+
 	p.X.Padding = 0
 	p.Y.Padding = 0
 	p.X.Max = 1.5

plotter/palettethumbnailer.go

@@ -0,0 +1,67 @@
+// Copyright ©2017 The gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package plotter
+
+import (
+	"image/color"
+
+	"github.com/gonum/plot"
+	"github.com/gonum/plot/palette"
+	"github.com/gonum/plot/vg"
+	"github.com/gonum/plot/vg/draw"
+)
+
+// PaletteThumbnailers creates a group of objects that can be used to
+// add legend entries for the colors in a color palette.
+func PaletteThumbnailers(p palette.Palette) []plot.Thumbnailer {
+	colors := p.Colors()
+	thumbnailers := make([]plot.Thumbnailer, len(colors))
+	for i, c := range colors {
+		thumbnailers[i] = paletteThumbnailer{color: c}
+	}
+	return thumbnailers
+}
+
+// paletteThumbnailer implements the Thumbnailer interface
+// for color palettes.
+type paletteThumbnailer struct {
+	color color.Color
+}
+
+// Thumbnail fulfills the plot.Thumbnailer interface.
+func (t paletteThumbnailer) Thumbnail(c *draw.Canvas) {
+	lineStyle := draw.LineStyle{
+		Color:    color.Black,
+		Width:    vg.Points(1),
+		Dashes:   []vg.Length{},
+		DashOffs: 0,
+	}
+
+	// Here we draw the fill.
+	pts := []vg.Point{
+		{X: c.Min.X, Y: c.Min.Y},
+		{X: c.Min.X, Y: c.Max.Y},
+		{X: c.Max.X, Y: c.Max.Y},
+		{X: c.Max.X, Y: c.Min.Y},
+	}
+	poly := c.ClipPolygonY(pts)
+	c.FillPolygon(t.color, poly)
+
+	// Here we draw the upper border.
+	pts = []vg.Point{
+		{X: c.Min.X, Y: c.Max.Y},
+		{X: c.Max.X, Y: c.Max.Y},
+	}
+	outline := c.ClipLinesY(pts)
+	c.StrokeLines(lineStyle, outline...)
+
+	// Here we draw the lower border.
+	pts = []vg.Point{
+		{X: c.Min.X, Y: c.Min.Y},
+		{X: c.Max.X, Y: c.Min.Y},
+	}
+	outline = c.ClipLinesY(pts)
+	c.StrokeLines(lineStyle, outline...)
+}