Added bar chart example with positive and negative values

plotter/barchart_test.go

@@ -7,6 +7,7 @@ package plotter
 import (
 	"image/color"
 	"log"
+	"math/rand"
 	"testing"
 
 	"github.com/gonum/plot"
@@ -172,3 +173,102 @@ func TestBarChart(t *testing.T) {
 		"horizontalBarChart.png", "barChart2.png",
 		"stackedBarChart.png")
 }
+
+// This example shows a bar chart with both positive and negative values.
+func ExampleBarChart_positiveNegative() {
+	rnd := rand.New(rand.NewSource(1))
+
+	// Create random data points between -1 and 1.
+	const n = 6
+	data1 := make(Values, n)
+	data2 := make(Values, n)
+	net := make(XYs, n) // net = data1 + data2
+	for i := 0; i < n; i++ {
+		data1[i] = rnd.Float64()*2 - 1
+		data2[i] = rnd.Float64()*2 - 1
+		net[i].X = data1[i] + data2[i]
+		net[i].Y = float64(i)
+	}
+
+	// posNeg splits an array into two arrays containing the positive and
+	// negative values, respectively, from the original array.
+	posNeg := func(d Values) (pos, neg Values) {
+		pos = make(Values, len(d))
+		neg = make(Values, len(d))
+		for i, v := range d {
+			if v > 0 {
+				pos[i] = v
+			} else {
+				neg[i] = v
+			}
+		}
+		return
+	}
+
+	data1Pos, data1Neg := posNeg(data1)
+	data2Pos, data2Neg := posNeg(data2)
+
+	const barWidth = 0.3 * vg.Centimeter
+	pos1, err := NewBarChart(data1Pos, barWidth)
+	if err != nil {
+		log.Panic(err)
+	}
+	pos2, err := NewBarChart(data2Pos, barWidth)
+	if err != nil {
+		log.Panic(err)
+	}
+	neg1, err := NewBarChart(data1Neg, barWidth)
+	if err != nil {
+		log.Panic(err)
+	}
+	neg2, err := NewBarChart(data2Neg, barWidth)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	netDots, err := NewScatter(net)
+	if err != nil {
+		log.Panic(err)
+	}
+	netDots.Radius = vg.Points(1.25)
+
+	pos2.StackOn(pos1) // Specify that pos2 goes on top of pos1.
+	neg2.StackOn(neg1) // Specify that neg2 goes on top of neg1.
+
+	color1 := color.NRGBA{R: 112, G: 22, B: 0, A: 255}
+	color2 := color.NRGBA{R: 91, G: 194, B: 54, A: 100}
+
+	pos1.Color, neg1.Color = color1, color1
+	pos2.Color, neg2.Color = color2, color2
+
+	// Specify that we want a horizontal bar chart.
+	pos1.Horizontal, pos2.Horizontal, neg1.Horizontal, neg2.Horizontal = true, true, true, true
+
+	// Create a line at zero.
+	zero, err := NewLine(XYs{{0, 0}, {0, 5}})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	p, err := plot.New()
+	if err != nil {
+		log.Panic(err)
+	}
+	p.Add(zero, pos1, pos2, neg1, neg2, netDots)
+	p.NominalY("Alpha", "Bravo", "Charlie", "Echo", "Foxtrot", "Golf")
+
+	p.Legend.Add("1", pos1)
+	p.Legend.Add("2", pos2)
+	p.Legend.Add("Sum", netDots)
+	p.Legend.Left = true
+	p.Legend.ThumbnailWidth = 2 * vg.Millimeter
+
+	err = p.Save(100, 100, "testdata/barChart_positiveNegative.png")
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+func TestBarChart_positiveNegative(t *testing.T) {
+	cmpimg.CheckPlot(ExampleBarChart_positiveNegative, t, "barChart_positiveNegative.png")
+}