sane default handling

chart.go

@@ -354,13 +354,13 @@ func (c Chart) getValueFormatters() (x, y, ya ValueFormatter) {
 		}
 	}
 	if c.XAxis.ValueFormatter != nil {
-		x = c.XAxis.ValueFormatter
+		x = c.XAxis.GetValueFormatter()
 	}
 	if c.YAxis.ValueFormatter != nil {
-		y = c.YAxis.ValueFormatter
+		y = c.YAxis.GetValueFormatter()
 	}
 	if c.YAxisSecondary.ValueFormatter != nil {
-		ya = c.YAxisSecondary.ValueFormatter
+		ya = c.YAxisSecondary.GetValueFormatter()
 	}
 	return
 }

tick.go

@@ -47,7 +47,7 @@ func (t Ticks) String() string {
 // GenerateContinuousTicks generates a set of ticks.
 func GenerateContinuousTicks(r Renderer, ra Range, isVertical bool, style Style, vf ValueFormatter) []Tick {
 	if vf == nil {
-		panic("vf is nil; did you remember to set a default value formatter?")
+		vf = FloatValueFormatter
 	}
 
 	var ticks []Tick

xaxis.go

@@ -30,6 +30,14 @@ func (xa XAxis) GetStyle() Style {
 	return xa.Style
 }
 
+// GetValueFormatter returns the value formatter for the axis.
+func (xa XAxis) GetValueFormatter() ValueFormatter {
+	if xa.ValueFormatter != nil {
+		return xa.ValueFormatter
+	}
+	return FloatValueFormatter
+}
+
 // GetTickPosition returns the tick position option for the axis.
 func (xa XAxis) GetTickPosition(defaults ...TickPosition) TickPosition {
 	if xa.TickPosition == TickPositionUnset {

yaxis.go

@@ -41,6 +41,14 @@ func (ya YAxis) GetStyle() Style {
 	return ya.Style
 }
 
+// GetValueFormatter returns the value formatter for the axis.
+func (ya YAxis) GetValueFormatter() ValueFormatter {
+	if ya.ValueFormatter != nil {
+		return ya.ValueFormatter
+	}
+	return FloatValueFormatter
+}
+
 // GetTickStyle returns the tick style.
 func (ya YAxis) GetTickStyle() Style {
 	return ya.TickStyle