adding validation.

time_series.go

@@ -1,6 +1,9 @@
 package chart
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
 
 // TimeSeries is a line on a chart.
 type TimeSeries struct {
@@ -59,3 +62,15 @@ func (ts TimeSeries) Render(r Renderer, canvasBox Box, xrange, yrange Range, def
 	style := ts.Style.InheritFrom(defaults)
 	Draw.LineSeries(r, canvasBox, xrange, yrange, style, ts)
 }
+
+// Validate validates the series.
+func (ts TimeSeries) Validate() error {
+	if len(ts.XValues) == 0 {
+		return fmt.Errorf("time series must have xvalues set")
+	}
+
+	if len(ts.YValues) == 0 {
+		return fmt.Errorf("time series must have yvalues set")
+	}
+	return nil
+}