plotter: fix handling of x/y-offsets in Labels glyph boxes

This CL fixes the handling of the X/Y-offsets of plotter.Labels: they
were previously ignored in the glyph box computation.
This CL also adds a test that exercizes offsetted labels and their glyph box.

Fixes #710.

plotter/labels.go

@@ -95,10 +95,11 @@ func (l *Labels) DataRange() (xmin, xmax, ymin, ymax float64) {
 func (l *Labels) GlyphBoxes(p *plot.Plot) []plot.GlyphBox {
 	bs := make([]plot.GlyphBox, len(l.Labels))
 	for i, label := range l.Labels {
-		bs[i].X = p.X.Norm(l.XYs[i].X)
-		bs[i].Y = p.Y.Norm(l.XYs[i].Y)
-		sty := l.TextStyle[i]
-		bs[i].Rectangle = sty.Rectangle(label)
+		bs[i] = plot.GlyphBox{
+			X:         p.X.Norm(l.XYs[i].X),
+			Y:         p.Y.Norm(l.XYs[i].Y),
+			Rectangle: l.TextStyle[i].Rectangle(label).Add(l.Offset),
+		}
 	}
 	return bs
 }

plotter/labels_example_test.go

@@ -31,8 +31,7 @@ func ExampleLabels() {
 			{X: -5, Y: 15},
 		},
 		Labels: []string{"A", "B", "C", "D"},
-	},
-	)
+	})
 	if err != nil {
 		log.Fatalf("could not creates labels plotter: %+v", err)
 	}

plotter/labels_test.go

@@ -102,6 +102,36 @@ func TestLabelsWithGlyphBoxes(t *testing.T) {
 				sty.YAlign = draw.YBottom
 			}
 
+			// label with positive x/y-offsets
+			loffp, err := plotter.NewLabels(plotter.XYLabels{
+				XYs:    []plotter.XY{{X: left}},
+				Labels: []string{"Bg"},
+			})
+			if err != nil {
+				t.Fatalf("could not creates labels plotter: %+v", err)
+			}
+			for i := range loffp.TextStyle {
+				sty := &loffp.TextStyle[i]
+				sty.Font.Size = vg.Length(fontSize)
+				sty.Color = color.RGBA{G: 255, A: 255}
+			}
+			loffp.Offset = vg.Point{X: 75, Y: 75}
+
+			// label with negative x/y-offsets
+			loffm, err := plotter.NewLabels(plotter.XYLabels{
+				XYs:    []plotter.XY{{X: left}},
+				Labels: []string{"Bg"},
+			})
+			if err != nil {
+				t.Fatalf("could not creates labels plotter: %+v", err)
+			}
+			for i := range loffm.TextStyle {
+				sty := &loffm.TextStyle[i]
+				sty.Font.Size = vg.Length(fontSize)
+				sty.Color = color.RGBA{B: 255, A: 255}
+			}
+			loffm.Offset = vg.Point{X: -40, Y: -40}
+
 			m5 := plotter.NewFunction(func(float64) float64 { return -0.5 })
 			m5.LineStyle.Color = color.RGBA{R: 255, A: 255}
 
@@ -111,7 +141,7 @@ func TestLabelsWithGlyphBoxes(t *testing.T) {
 			p5 := plotter.NewFunction(func(float64) float64 { return +0.5 })
 			p5.LineStyle.Color = color.RGBA{B: 255, A: 255}
 
-			p.Add(labels, lred, m5, l0, p5)
+			p.Add(labels, lred, m5, l0, p5, loffp, loffm)
 			p.Add(plotter.NewGrid())
 			p.Add(plotter.NewGlyphBoxes())