fixing range validation and tests.

chart.go

@@ -295,6 +295,9 @@ func (c Chart) checkRanges(xr, yr, yra Range) error {
 	if math.IsNaN(xDelta) {
 		return errors.New("nan x-range delta")
 	}
+	if xDelta == 0 {
+		return errors.New("zero x-range delta; there needs to be at least (2) values")
+	}
 
 	yDelta := yr.GetDelta()
 	if math.IsInf(yDelta, 0) {

chart_test.go

@@ -422,5 +422,64 @@ func TestChartValidatesSeries(t *testing.T) {
 	}
 
 	assert.NotNil(c.validateSeries())
-
+}
+
+func TestChartCheckRanges(t *testing.T) {
+	assert := assert.New(t)
+
+	c := Chart{
+		Series: []Series{
+			ContinuousSeries{
+				XValues: []float64{1.0, 2.0},
+				YValues: []float64{3.10, 3.14},
+			},
+		},
+	}
+
+	xr, yr, yra := c.getRanges()
+	assert.Nil(c.checkRanges(xr, yr, yra))
+}
+
+func TestChartCheckRangesFailure(t *testing.T) {
+	assert := assert.New(t)
+
+	c := Chart{
+		Series: []Series{
+			ContinuousSeries{
+				XValues: []float64{1.0, 2.0},
+				YValues: []float64{3.14, 3.14},
+			},
+		},
+	}
+
+	xr, yr, yra := c.getRanges()
+	assert.NotNil(c.checkRanges(xr, yr, yra))
+}
+
+func TestChartCheckRangesWithRanges(t *testing.T) {
+	assert := assert.New(t)
+
+	c := Chart{
+		XAxis: XAxis{
+			Range: &ContinuousRange{
+				Min: 0,
+				Max: 10,
+			},
+		},
+		YAxis: YAxis{
+			Range: &ContinuousRange{
+				Min: 0,
+				Max: 5,
+			},
+		},
+		Series: []Series{
+			ContinuousSeries{
+				XValues: []float64{1.0, 2.0},
+				YValues: []float64{3.14, 3.14},
+			},
+		},
+	}
+
+	xr, yr, yra := c.getRanges()
+	assert.Nil(c.checkRanges(xr, yr, yra))
 }