How to create a heatMap legend displaying the gradient scaling ?

[chmike]

How do I create a heatmap legend on the right as in this matplotlib example below ? Is this possible without creating a second graphic ? Minimum coding would be preferable. Being able to adjust the hight of the color gradient bar would be also desirable.

[kortschak]

This is demonstrated in the heatmap example here.

[chmike]

Thanks, but its not very convenient to use. For instance, it doesn't work with a 256 colors palette.
One would need to scale the thumbnail and provide multiple values to get something equivalent to the given example.
How do one scale the thumbnail ?

[chmike]

Also, it isn't obvious how to convert the example to generate an SVG or pdf output file.

[sbinet]

here is what a modified hplot/h2d example gives:

here is the simple modification:

diff --git a/hplot/h2d_example_test.go b/hplot/h2d_example_test.go
index 661acf1d..92422315 100644
--- a/hplot/h2d_example_test.go
+++ b/hplot/h2d_example_test.go
@@ -12,6 +12,7 @@ import (
 	"golang.org/x/exp/rand"
 	"gonum.org/v1/gonum/mat"
 	"gonum.org/v1/gonum/stat/distmv"
+	"gonum.org/v1/plot/palette"
 	"gonum.org/v1/plot/plotter"
 	"gonum.org/v1/plot/vg"
 )
@@ -72,7 +73,8 @@ func ExampleH2D_withLegend() {
 		v = dist.Rand(v)
 		h2d.Fill(v[0], v[1], 1)
 	}
-	h := hplot.NewH2D(h2d, nil)
+	pal := palette.Rainbow(50, palette.Yellow, palette.Green, 1, 0.5, 0.5)
+	h := hplot.NewH2D(h2d, pal)
 
 	p := hplot.New()
 	p.Title.Text = "Hist-2D"
 }

ie:

func ExampleH2D_withLegend() {
	h2d := hbook.NewH2D(100, -10, 10, 100, -10, 10)

	const npoints = 10000

	dist, ok := distmv.NewNormal(
		[]float64{0, 1},
		mat.NewSymDense(2, []float64{4, 0, 0, 2}),
		rand.New(rand.NewSource(1234)),
	)
	if !ok {
		log.Fatalf("error creating distmv.Normal")
	}

	v := make([]float64, 2)
	// Draw some random values from the standard
	// normal distribution.
	for i := 0; i < npoints; i++ {
		v = dist.Rand(v)
		h2d.Fill(v[0], v[1], 1)
	}
	pal := palette.Rainbow(50, palette.Yellow, palette.Green, 1, 0.5, 0.5)
	h := hplot.NewH2D(h2d, pal)

	p := hplot.New()
	p.Title.Text = "Hist-2D"
	p.X.Label.Text = "x"
	p.Y.Label.Text = "y"

	p.Add(h)
	p.Add(plotter.NewGrid())

	fig := hplot.Figure(p, hplot.WithLegend(h.Legend()))
	err := hplot.Save(fig, 20*vg.Centimeter, 20*vg.Centimeter, "testdata/h2d_plot_legend.png")
	if err != nil {
		log.Fatal(err)
	}
}

modifiying for a SVG is relatively trivial:

diff --git a/hplot/h2d_example_test.go b/hplot/h2d_example_test.go
index 661acf1d..b13aa8e5 100644
--- a/hplot/h2d_example_test.go
+++ b/hplot/h2d_example_test.go
@@ -12,6 +12,7 @@ import (
 	"golang.org/x/exp/rand"
 	"gonum.org/v1/gonum/mat"
 	"gonum.org/v1/gonum/stat/distmv"
+	"gonum.org/v1/plot/palette"
 	"gonum.org/v1/plot/plotter"
 	"gonum.org/v1/plot/vg"
 )
@@ -72,7 +73,8 @@ func ExampleH2D_withLegend() {
 		v = dist.Rand(v)
 		h2d.Fill(v[0], v[1], 1)
 	}
-	h := hplot.NewH2D(h2d, nil)
+	pal := palette.Rainbow(50, palette.Yellow, palette.Green, 1, 0.5, 0.5)
+	h := hplot.NewH2D(h2d, pal)
 
 	p := hplot.New()
 	p.Title.Text = "Hist-2D"
@@ -83,8 +85,12 @@ func ExampleH2D_withLegend() {
 	p.Add(plotter.NewGrid())
 
 	fig := hplot.Figure(p, hplot.WithLegend(h.Legend()))
-	err := hplot.Save(fig, 10*vg.Centimeter, 10*vg.Centimeter, "testdata/h2d_plot_legend.png")
+	err := hplot.Save(fig, 20*vg.Centimeter, 20*vg.Centimeter, "testdata/h2d_plot_legend.svg")
 	if err != nil {
 		log.Fatal(err)
 	}
 }

[sbinet]

a smarter plotter.PaletteThumbnailers (or different) could compute the available vertical headroom, adapt each ladder dimension and take the whole space.

(here, I cheated and chose 50 as the number of different colors so the legend would take the whole vertical space)