add logarithmic axes support, with tests. Supports positive Y-values only. (#141)

Co-authored-by: Ton Wessling <twessling@ebay.com>

examples/logarithmic_axes/main.go

@@ -0,0 +1,41 @@
+package main
+
+//go:generate go run main.go
+
+import (
+	"os"
+
+	"github.com/wcharczuk/go-chart"
+)
+
+func main() {
+
+	/*
+	   In this example we set the primary YAxis to have logarithmic range.
+	*/
+
+	graph := chart.Chart{
+		Background: chart.Style{
+			Padding: chart.Box{
+				Top:  20,
+				Left: 20,
+			},
+		},
+		Series: []chart.Series{
+			chart.ContinuousSeries{
+				Name:    "A test series",
+				XValues: []float64{1.0, 2.0, 3.0, 4.0, 5.0},
+				YValues: []float64{1, 10, 100, 1000, 10000},
+			},
+		},
+		YAxis: chart.YAxis{
+			Style:     chart.Shown(),
+			NameStyle: chart.Shown(),
+			Range: &chart.LogarithmicRange{},
+		},
+	}
+
+	f, _ := os.Create("output.png")
+	defer f.Close()
+	graph.Render(chart.PNG, f)
+}