all: add AutoRescale to Axis

This CL adds a new field to Axis to allow for a better interaction
between a plot, an axis and its ticker, in order to automatically adapt
the range of a given axis.

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

axis.go

@@ -88,6 +88,10 @@ type Axis struct {
 	// to the normalized coordinate system of the axis—its distance
 	// along the axis as a fraction of the axis range.
 	Scale Normalizer
+
+	// AutoRescale enables an axis to automatically adapt its minimum
+	// and maximum boundaries, according to its underlying Ticker.
+	AutoRescale bool
 }
 
 // makeAxis returns a default Axis.
@@ -161,6 +165,14 @@ func (a *Axis) sanitizeRange() {
 		a.Min--
 		a.Max++
 	}
+
+	if a.AutoRescale {
+		marks := a.Tick.Marker.Ticks(a.Min, a.Max)
+		for _, t := range marks {
+			a.Min = math.Min(a.Min, t.Value)
+			a.Max = math.Max(a.Max, t.Value)
+		}
+	}
 }
 
 // LinearScale an be used as the value of an Axis.Scale function to

plotter/logscale_example_test.go

@@ -41,3 +41,34 @@ func Example_logScale() {
 		log.Panic(err)
 	}
 }
+
+// Example_logScaleWithAutoRescale shows how to create a plot with a log-scale on the Y-axis.
+// The Y-axis is instructed to automatically adapt its range according to
+// the underlying Y-ticker.
+func Example_logScaleWithAutoRescale() {
+	p := plot.New()
+	p.Title.Text = "My Plot"
+	p.Y.Scale = plot.LogScale{}
+	p.Y.Tick.Marker = plot.LogTicks{Prec: -1}
+	p.Y.AutoRescale = true
+	p.X.Label.Text = "x"
+	p.Y.Label.Text = "f(x)"
+
+	f := plotter.NewFunction(math.Exp)
+	f.XMin = 0.2
+	f.XMax = 10
+	f.Color = color.RGBA{R: 255, A: 255}
+
+	p.Add(f, plotter.NewGrid())
+	p.Legend.Add("exp(x)", f)
+
+	p.X.Min = f.XMin
+	p.X.Max = f.XMax
+	p.Y.Min = math.Exp(f.XMin)
+	p.Y.Max = math.Exp(f.XMax)
+
+	err := p.Save(10*vg.Centimeter, 10*vg.Centimeter, "testdata/logscale_autorescale.png")
+	if err != nil {
+		log.Panic(err)
+	}
+}

plotter/logscale_test.go

@@ -13,3 +13,7 @@ import (
 func TestLogScale(t *testing.T) {
 	cmpimg.CheckPlot(Example_logScale, t, "logscale.png")
 }
+
+func TestLogScaleWithAutoRescale(t *testing.T) {
+	cmpimg.CheckPlot(Example_logScaleWithAutoRescale, t, "logscale_autorescale.png")
+}