plot: fix handling of font descents

Author: kortschak

axis.go

@@ -262,20 +262,21 @@ func (a horizontalAxis) draw(c draw.Canvas) {
 		x -= a.Label.Font.Width(a.Label.Text) / 2
 	}
 	if a.Label.Text != "" {
-		y -= a.Label.Font.Extents().Descent
-		c.FillText(a.Label.TextStyle, vg.Point{X: x, Y: y}, a.Label.Text)
+		descent := a.Label.Font.Extents().Descent
+		c.FillText(a.Label.TextStyle, vg.Point{X: x, Y: y - descent}, a.Label.Text)
 		y += a.Label.Height(a.Label.Text)
 		y += a.Label.Padding
 	}
 
 	marks := a.Tick.Marker.Ticks(a.Min, a.Max)
 	ticklabelheight := tickLabelHeight(a.Tick.Label, marks)
+	descent := a.Tick.Label.Font.Extents().Descent
 	for _, t := range marks {
 		x := c.X(a.Norm(t.Value))
 		if !c.ContainsX(x) || t.IsMinor() {
 			continue
 		}
-		c.FillText(a.Tick.Label, vg.Point{X: x, Y: y + ticklabelheight}, t.Label)
+		c.FillText(a.Tick.Label, vg.Point{X: x, Y: y + ticklabelheight - descent}, t.Label)
 	}
 
 	if len(marks) > 0 {
@@ -362,8 +363,9 @@ func (a verticalAxis) draw(c draw.Canvas) {
 			y = c.Max.Y
 			y -= a.Label.Font.Width(a.Label.Text) / 2
 		}
-		c.FillText(sty, vg.Point{X: x, Y: y}, a.Label.Text)
-		x += -a.Label.Font.Extents().Descent
+		descent := a.Label.Font.Extents().Descent
+		c.FillText(sty, vg.Point{X: x + descent, Y: y}, a.Label.Text)
+		x -= descent
 		x += a.Label.Padding
 	}
 	marks := a.Tick.Marker.Ticks(a.Min, a.Max)
@@ -372,12 +374,13 @@ func (a verticalAxis) draw(c draw.Canvas) {
 	}
 
 	major := false
+	descent := a.Tick.Label.Font.Extents().Descent
 	for _, t := range marks {
 		y := c.Y(a.Norm(t.Value))
 		if !c.ContainsY(y) || t.IsMinor() {
 			continue
 		}
-		c.FillText(a.Tick.Label, vg.Point{X: x, Y: y}, t.Label)
+		c.FillText(a.Tick.Label, vg.Point{X: x, Y: y - descent}, t.Label)
 		major = true
 	}
 	if major {

legend.go

@@ -105,7 +105,8 @@ func (l *Legend) Draw(c draw.Canvas) {
 	textx += l.XOffs
 	iconx += l.XOffs
 
-	enth := l.entryHeight()
+	descent := sty.Font.Extents().Descent
+	enth := l.entryHeight() + descent
 	y := c.Max.Y - enth
 	if !l.Top {
 		y = c.Min.Y + (enth+l.Padding)*(vg.Length(len(l.entries))-1)
@@ -124,7 +125,7 @@ func (l *Legend) Draw(c draw.Canvas) {
 		panic("plot: invalid vertical offset for the legend's entries")
 	}
 	yoff := vg.Length(l.YPosition-draw.PosBottom) / 2
-	yoff *= -sty.Font.Extents().Descent
+	yoff -= descent
 
 	for _, e := range l.entries {
 		for _, t := range e.thumbs {

palette/moreland/testdata/moreland_golden.png

@@ -152,7 +152,8 @@ func (p *Plot) Draw(c draw.Canvas) {
 		c.Fill(c.Rectangle.Path())
 	}
 	if p.Title.Text != "" {
-		c.FillText(p.Title.TextStyle, vg.Point{X: c.Center().X, Y: c.Max.Y}, p.Title.Text)
+		descent := p.Title.TextStyle.Font.Extents().Descent
+		c.FillText(p.Title.TextStyle, vg.Point{X: c.Center().X, Y: c.Max.Y - descent}, p.Title.Text)
 		_, h, d := p.Title.Handler.Box(p.Title.Text, p.Title.Font)
 		c.Max.Y -= h + d
 		c.Max.Y -= p.Title.Padding