plot,vg/draw: add ability to set the axis label position

Fixes #589.

Author: sbinet

align_test.go

@@ -13,3 +13,7 @@ import (
 func TestAlign(t *testing.T) {
 	cmpimg.CheckPlot(ExampleAlign, t, "align.png")
 }
+
+func TestAxisLabels(t *testing.T) {
+	cmpimg.CheckPlot(ExampleAxis_labelsPosition, t, "axis_labels.png")
+}

axis.go

@@ -47,6 +47,13 @@ type Axis struct {
 		// counterclockwise will be added to the label
 		// text before drawing.
 		draw.TextStyle
+
+		// Position is where the axis label string should be drawn.
+		// The default value is draw.PosCenter, displaying the label
+		// at the center of the axis.
+		// Valid values are [-1,+1], with +1 being the far right/top
+		// of the axis, and -1 the far left/bottom of the axis.
+		Position float64
 	}
 
 	// LineStyle is the style of the axis line.
@@ -112,6 +119,7 @@ func makeAxis(o orientation) (Axis, error) {
 		XAlign: draw.XCenter,
 		YAlign: draw.YBottom,
 	}
+	a.Label.Position = draw.PosCenter
 
 	var (
 		xalign draw.XAlignment
@@ -240,10 +248,20 @@ func (a horizontalAxis) size() (h vg.Length) {
 
 // draw draws the axis along the lower edge of a draw.Canvas.
 func (a horizontalAxis) draw(c draw.Canvas) {
-	y := c.Min.Y
+	var (
+		x vg.Length
+		y = c.Min.Y
+	)
+	switch a.Label.Position {
+	case draw.PosCenter:
+		x = c.Center().X
+	case draw.PosRight:
+		x = c.Max.X
+		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: c.Center().X, Y: y}, a.Label.Text)
+		c.FillText(a.Label.TextStyle, vg.Point{X: x, Y: y}, a.Label.Text)
 		y += a.Label.Height(a.Label.Text)
 		y += a.Label.Padding
 	}
@@ -327,12 +345,22 @@ func (a verticalAxis) size() (w vg.Length) {
 
 // draw draws the axis along the left side of a draw.Canvas.
 func (a verticalAxis) draw(c draw.Canvas) {
-	x := c.Min.X
+	var (
+		x = c.Min.X
+		y vg.Length
+	)
 	if a.Label.Text != "" {
 		sty := a.Label.TextStyle
 		sty.Rotation += math.Pi / 2
 		x += a.Label.Height(a.Label.Text)
-		c.FillText(sty, vg.Point{X: x, Y: c.Center().Y}, a.Label.Text)
+		switch a.Label.Position {
+		case draw.PosCenter:
+			y = c.Center().Y
+		case draw.PosTop:
+			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
 		x += a.Label.Padding
 	}

example_align_test.go

@@ -5,6 +5,7 @@
 package plot_test
 
 import (
+	"log"
 	"math"
 	"os"
 
@@ -90,3 +91,25 @@ func ExampleAlign() {
 		panic(err)
 	}
 }
+
+func ExampleAxis_labelsPosition() {
+	p, err := plot.New()
+	if err != nil {
+		log.Fatalf("could not create plot: %+v", err)
+	}
+
+	p.Title.Text = "Title"
+	p.X.Label.Text = "X [mm]"
+	p.Y.Label.Text = "Y [A.U.]"
+	p.X.Label.Position = draw.PosRight
+	p.Y.Label.Position = draw.PosTop
+	p.X.Min = -10
+	p.X.Max = +10
+	p.Y.Min = -10
+	p.Y.Max = +10
+
+	err = p.Save(10*vg.Centimeter, 10*vg.Centimeter, "testdata/axis_labels.png")
+	if err != nil {
+		log.Fatalf("could not save plot: %+v", err)
+	}
+}

testdata/axis_labels_golden.png

@@ -71,6 +71,15 @@ const (
 	YBottom YAlignment = 0
 )
 
+// Position specifies the text position.
+const (
+	PosLeft   = -1
+	PosBottom = -1
+	PosCenter = 0
+	PosTop    = +1
+	PosRight  = +1
+)
+
 // LineStyle describes what a line will look like.
 type LineStyle struct {
 	// Color is the color of the line.