plot,plotter: properly handle Log-Y scale in Histogram

Fixes #377.

Previously, the Histogram.Plot and .DataRange method would implicitly
assume the bottom of a histogram would be 0.
This didn't pan out very well for Log-Y scale plots.

axis.go

@@ -94,7 +94,7 @@ func makeAxis(orientation bool) (Axis, error) {
 	}
 
 	a := Axis{
-		Min: math.Inf(1),
+		Min: math.Inf(+1),
 		Max: math.Inf(-1),
 		LineStyle: draw.LineStyle{
 			Color: color.Black,
@@ -469,7 +469,7 @@ var _ Ticker = LogTicks{}
 
 // Ticks returns Ticks in a specified range
 func (LogTicks) Ticks(min, max float64) []Tick {
-	if min <= 0 {
+	if min <= 0 || max <= 0 {
 		panic("Values must be greater than 0 for a log scale.")
 	}
 

plotter/histogram.go

@@ -32,6 +32,14 @@ type Histogram struct {
 	// LineStyle is the style of the outline of each
 	// bar of the histogram.
 	draw.LineStyle
+
+	// LogY allows rendering with a log-scaled Y axis.
+	// When enabled, histogram bins with no entries will be discarded from
+	// the histogram's DataRange.
+	// The lowest Y value for the DataRange will be corrected to leave an
+	// arbitrary amount of height for the smallest bin entry so it is visible
+	// on the final plot.
+	LogY bool
 }
 
 // NewHistogram returns a new histogram
@@ -77,25 +85,34 @@ func (h *Histogram) Plot(c draw.Canvas, p *plot.Plot) {
 	trX, trY := p.Transforms(&c)
 
 	for _, bin := range h.Bins {
+		ymin := c.Min.Y
+		ymax := c.Min.Y
+		if 0 != bin.Weight {
+			ymax = trY(bin.Weight)
+		}
+		xmin := trX(bin.Min)
+		xmax := trX(bin.Max)
 		pts := []vg.Point{
-			{trX(bin.Min), trY(0)},
-			{trX(bin.Max), trY(0)},
-			{trX(bin.Max), trY(bin.Weight)},
-			{trX(bin.Min), trY(bin.Weight)},
+			{xmin, ymin},
+			{xmax, ymin},
+			{xmax, ymax},
+			{xmin, ymax},
 		}
 		if h.FillColor != nil {
 			c.FillPolygon(h.FillColor, c.ClipPolygonXY(pts))
 		}
-		pts = append(pts, vg.Point{X: trX(bin.Min), Y: trY(0)})
+		pts = append(pts, vg.Point{X: xmin, Y: ymin})
 		c.StrokeLines(h.LineStyle, c.ClipLinesXY(pts)...)
 	}
 }
 
 // DataRange returns the minimum and maximum X and Y values
 func (h *Histogram) DataRange() (xmin, xmax, ymin, ymax float64) {
-	xmin = math.Inf(1)
+	xmin = math.Inf(+1)
 	xmax = math.Inf(-1)
+	ymin = math.Inf(+1)
 	ymax = math.Inf(-1)
+	ylow := math.Inf(+1) // ylow will hold the smallest non-zero y value.
 	for _, bin := range h.Bins {
 		if bin.Max > xmax {
 			xmax = bin.Max
@@ -106,6 +123,21 @@ func (h *Histogram) DataRange() (xmin, xmax, ymin, ymax float64) {
 		if bin.Weight > ymax {
 			ymax = bin.Weight
 		}
+		if bin.Weight < ymin {
+			ymin = bin.Weight
+		}
+		if bin.Weight != 0 && bin.Weight < ylow {
+			ylow = bin.Weight
+		}
+	}
+	switch h.LogY {
+	case true:
+		if ymin == 0 && !math.IsInf(ylow, +1) {
+			// Reserve a bit of space for the smallest bin to be displayed still.
+			ymin = ylow * 0.5
+		}
+	default:
+		ymin = 0
 	}
 	return
 }

plotter/histogram_test.go

@@ -94,3 +94,51 @@ func TestSingletonHistogram(t *testing.T) {
 	case <-done:
 	}
 }
+
+func ExampleHistogram_logScaleY() {
+	p, err := plot.New()
+	if err != nil {
+		log.Panic(err)
+	}
+	p.Title.Text = "Histogram in log-y"
+	p.Y.Scale = plot.LogScale{}
+	p.Y.Tick.Marker = plot.LogTicks{}
+	p.Y.Label.Text = "Y"
+	p.X.Label.Text = "X"
+
+	h1, err := NewHist(Values{
+		-2, -2,
+		-1,
+		+3, +3, +3, +3,
+		+1, +1, +1, +1, +1, +1, +1, +1, +1, +1,
+		+1, +1, +1, +1, +1, +1, +1, +1, +1, +1,
+	}, 16)
+	if err != nil {
+		log.Fatal(err)
+	}
+	h1.LogY = true
+	h1.FillColor = color.RGBA{255, 0, 0, 255}
+
+	h2, err := NewHist(Values{
+		-3, -3, -3,
+		+2, +2, +2, +2, +2,
+	}, 16)
+
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	h2.LogY = true
+	h2.FillColor = color.RGBA{0, 0, 255, 255}
+
+	p.Add(h1, h2, NewGrid())
+
+	err = p.Save(200, 200, "testdata/histogram_logy.png")
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func TestHistogramLogScale(t *testing.T) {
+	cmpimg.CheckPlot(ExampleHistogram_logScaleY, t, "histogram_logy.png")
+}