plot: properly handle descent in axis labels glyph boxes

Fixes gonum#676.

Signed-off-by: Sebastien Binet <binet@cern.ch>

axis.go

@@ -296,14 +296,29 @@ func (a horizontalAxis) draw(c draw.Canvas) {
 
 // GlyphBoxes returns the GlyphBoxes for the tick labels.
 func (a horizontalAxis) GlyphBoxes(*Plot) []GlyphBox {
-	var boxes []GlyphBox
-	for _, t := range a.Tick.Marker.Ticks(a.Min, a.Max) {
+	var (
+		boxes []GlyphBox
+		yoff  font.Length
+	)
+
+	if a.Label.Text != "" {
+		//		yoff += a.Label.TextStyle.FontExtents().Descent
+		yoff += a.Label.TextStyle.Height(a.Label.Text)
+		yoff += a.Label.Padding
+	}
+
+	var (
+		marks   = a.Tick.Marker.Ticks(a.Min, a.Max)
+		height  = tickLabelHeight(a.Tick.Label, marks)
+		descent = a.Tick.Label.FontExtents().Descent
+	)
+	for _, t := range marks {
 		if t.IsMinor() {
 			continue
 		}
 		box := GlyphBox{
 			X:         a.Norm(t.Value),
-			Rectangle: a.Tick.Label.Rectangle(t.Label),
+			Rectangle: a.Tick.Label.Rectangle(t.Label).Add(vg.Point{Y: yoff + height + descent}),
 		}
 		boxes = append(boxes, box)
 	}
@@ -397,14 +412,36 @@ func (a verticalAxis) draw(c draw.Canvas) {
 
 // GlyphBoxes returns the GlyphBoxes for the tick labels
 func (a verticalAxis) GlyphBoxes(*Plot) []GlyphBox {
-	var boxes []GlyphBox
-	for _, t := range a.Tick.Marker.Ticks(a.Min, a.Max) {
+	var (
+		boxes []GlyphBox
+		xoff  font.Length
+	)
+
+	if a.Label.Text != "" {
+		sty := a.Label.TextStyle
+		sty.Rotation += math.Pi / 2
+
+		xoff += a.Label.TextStyle.Height(a.Label.Text)
+		xoff += a.Label.TextStyle.FontExtents().Descent
+		xoff += a.Label.Padding
+	}
+
+	marks := a.Tick.Marker.Ticks(a.Min, a.Max)
+	if w := tickLabelWidth(a.Tick.Label, marks); len(marks) > 0 && w > 0 {
+		xoff += w
+	}
+
+	var (
+		ext  = a.Tick.Label.FontExtents()
+		desc = ext.Height - ext.Ascent
+	)
+	for _, t := range marks {
 		if t.IsMinor() {
 			continue
 		}
 		box := GlyphBox{
 			Y:         a.Norm(t.Value),
-			Rectangle: a.Tick.Label.Rectangle(t.Label),
+			Rectangle: a.Tick.Label.Rectangle(t.Label).Add(vg.Point{X: xoff, Y: desc}),
 		}
 		boxes = append(boxes, box)
 	}

legend.go

@@ -173,8 +173,12 @@ func (l *Legend) Rectangle(c draw.Canvas) vg.Rectangle {
 // entryHeight returns the height of the tallest legend
 // entry text.
 func (l *Legend) entryHeight() (height vg.Length) {
+	var (
+		ext  = l.TextStyle.FontExtents()
+		desc = ext.Height - ext.Ascent // descent + linegap
+	)
 	for _, e := range l.entries {
-		if h := l.TextStyle.Rectangle(e.text).Max.Y; h > height {
+		if h := l.TextStyle.Rectangle(e.text).Max.Y + desc; h > height {
 			height = h
 		}
 	}

plot.go

@@ -191,12 +191,46 @@ func (p *Plot) DataCanvas(da draw.Canvas) draw.Canvas {
 
 // DrawGlyphBoxes draws red outlines around the plot's
 // GlyphBoxes.  This is intended for debugging.
-func (p *Plot) DrawGlyphBoxes(c *draw.Canvas) {
-	c.SetColor(color.RGBA{R: 255, A: 255})
+func (p *Plot) DrawGlyphBoxes(c draw.Canvas) {
+	dac := p.DataCanvas(c)
+	sty := draw.LineStyle{
+		Color: color.RGBA{R: 255, A: 255},
+		Width: vg.Points(0.5),
+	}
+
+	drawBox := func(c draw.Canvas, b GlyphBox) {
+		x := c.X(b.X) + b.Rectangle.Min.X
+		y := c.Y(b.Y) + b.Rectangle.Min.Y
+		c.StrokeLines(sty, []vg.Point{
+			{X: x, Y: y},
+			{X: x + b.Rectangle.Size().X, Y: y},
+			{X: x + b.Rectangle.Size().X, Y: y + b.Rectangle.Size().Y},
+			{X: x, Y: y + b.Rectangle.Size().Y},
+			{X: x, Y: y},
+		})
+	}
+
 	for _, b := range p.GlyphBoxes(p) {
-		b.Rectangle.Min.X += c.X(b.X)
-		b.Rectangle.Min.Y += c.Y(b.Y)
-		c.Stroke(b.Rectangle.Path())
+		drawBox(dac, b)
+	}
+
+	p.X.sanitizeRange()
+	p.Y.sanitizeRange()
+
+	x := horizontalAxis{p.X}
+	y := verticalAxis{p.Y}
+
+	ywidth := y.size()
+	xheight := x.size()
+
+	cx := padX(p, draw.Crop(c, ywidth, 0, 0, 0))
+	for _, b := range x.GlyphBoxes(nil) {
+		drawBox(cx, b)
+	}
+
+	cy := padY(p, draw.Crop(c, 0, 0, xheight, 0))
+	for _, b := range y.GlyphBoxes(nil) {
+		drawBox(cy, b)
 	}
 }
 

plot_test.go

@@ -128,3 +128,59 @@ func TestIssue514(t *testing.T) {
 		})
 	}
 }
+
+func TestDrawGlyphBoxes(t *testing.T) {
+	cmpimg.CheckPlot(func() {
+		p := plot.New()
+
+		p.X.Min = 0
+		p.X.Max = 10
+		p.Y.Min = 0
+		p.Y.Max = 10
+
+		f1 := plotter.NewFunction(func(x float64) float64 { return 5 })
+		f1.LineStyle.Color = color.RGBA{R: 255, A: 255}
+
+		f2 := plotter.NewFunction(func(x float64) float64 { return 6 })
+		f2.LineStyle.Color = color.RGBA{B: 255, A: 255}
+
+		labels, err := plotter.NewLabels(plotter.XYLabels{
+			XYs: []plotter.XY{
+				{X: 2.5, Y: 2.5},
+				{X: 7.5, Y: 2.5},
+				{X: 7.5, Y: 7.5},
+				{X: 2.5, Y: 7.5},
+			},
+			Labels: []string{"Agg", "Bgg", "Cgg", "Dgg"},
+		})
+		if err != nil {
+			t.Fatalf("could not creates labels plotter: %+v", err)
+		}
+
+		p.Add(f1, f2, labels)
+		p.Add(plotter.NewGrid())
+
+		p.Legend.Add("fg1", f1)
+		p.Legend.Add("fg2", f2)
+		p.Legend.Top = true
+
+		c := vgimg.PngCanvas{
+			Canvas: vgimg.New(20*vg.Centimeter, 15*vg.Centimeter),
+		}
+
+		d := draw.New(c)
+		p.Draw(d)
+		p.DrawGlyphBoxes(d)
+
+		buf := new(bytes.Buffer)
+		_, err = c.WriteTo(buf)
+		if err != nil {
+			t.Fatalf("error: %+v", err)
+		}
+
+		err = ioutil.WriteFile("testdata/glyphbox.png", buf.Bytes(), 0644)
+		if err != nil {
+			t.Fatalf("could not save plot: %+v", err)
+		}
+	}, t, "glyphbox.png")
+}