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

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

logarithmic_range_test.go

@@ -0,0 +1,46 @@
+package chart
+
+import (
+	"testing"
+
+	"github.com/blend/go-sdk/assert"
+)
+
+func TestLogRangeTranslate(t *testing.T) {
+   	assert := assert.New(t)
+	values := []float64{1, 10, 100, 1000, 10000, 100000, 1000000}
+	r := LogarithmicRange{Domain: 1000}
+	r.Min, r.Max = MinMax(values...)
+
+	assert.Equal(0, r.Translate(0))       // goes to bottom
+	assert.Equal(0, r.Translate(1))       // goes to bottom
+	assert.Equal(160, r.Translate(10))    // roughly 1/6th of max
+	assert.Equal(500, r.Translate(1000))  // roughly 1/2 of max (1.0e6 / 1.0e3)
+	assert.Equal(1000, r.Translate(1000000)) // max value
+}
+
+func TestGetTicks(t *testing.T) {
+	assert := assert.New(t)
+	values := []float64{35, 512, 1525122}
+	r := LogarithmicRange{Domain: 1000}
+	r.Min, r.Max = MinMax(values...)
+
+	ticks := r.GetTicks(nil, Style{}, FloatValueFormatter)
+	assert.Equal(7, len(ticks))
+	assert.Equal(10, ticks[0].Value)
+	assert.Equal(100, ticks[1].Value)
+	assert.Equal(10000000, ticks[6].Value)
+}
+
+func TestGetTicksFromHigh(t *testing.T) {
+	assert := assert.New(t)
+	values := []float64{1412, 352144, 1525122} // min tick should be 1000
+	r := LogarithmicRange{}
+	r.Min, r.Max = MinMax(values...)
+
+	ticks := r.GetTicks(nil, Style{}, FloatValueFormatter)
+	assert.Equal(5, len(ticks))
+	assert.Equal(float64(1000), ticks[0].Value)
+	assert.Equal(float64(10000), ticks[1].Value)
+	assert.Equal(float64(10000000), ticks[4].Value)
+}