adding validation.

min_max_series.go

@@ -1,6 +1,9 @@
 package chart
 
-import "math"
+import (
+	"fmt"
+	"math"
+)
 
 // MinSeries draws a horizontal line at the minimum value of the inner series.
 type MinSeries struct {
@@ -60,6 +63,14 @@ func (ms *MinSeries) ensureMinValue() {
 	}
 }
 
+// Validate validates the series.
+func (ms *MinSeries) Validate() error {
+	if ms.InnerSeries == nil {
+		return fmt.Errorf("min series requires InnerSeries to be set")
+	}
+	return nil
+}
+
 // MaxSeries draws a horizontal line at the maximum value of the inner series.
 type MaxSeries struct {
 	Name        string
@@ -117,3 +128,11 @@ func (ms *MaxSeries) ensureMaxValue() {
 		ms.maxValue = &maxValue
 	}
 }
+
+// Validate validates the series.
+func (ms *MaxSeries) Validate() error {
+	if ms.InnerSeries == nil {
+		return fmt.Errorf("max series requires InnerSeries to be set")
+	}
+	return nil
+}