adding validation.

sma_series.go

@@ -1,5 +1,7 @@
 package chart
 
+import "fmt"
+
 const (
 	// DefaultSimpleMovingAveragePeriod is the default number of values to average.
 	DefaultSimpleMovingAveragePeriod = 16
@@ -88,3 +90,11 @@ func (sma SMASeries) Render(r Renderer, canvasBox Box, xrange, yrange Range, def
 	style := sma.Style.InheritFrom(defaults)
 	Draw.LineSeries(r, canvasBox, xrange, yrange, style, sma)
 }
+
+// Validate validates the series.
+func (sma SMASeries) Validate() error {
+	if sma.InnerSeries == nil {
+		return fmt.Errorf("sma series requires InnerSeries to be set")
+	}
+	return nil
+}