introduces the Range interface (instead of a concrete type).

concat_series.go

@@ -0,0 +1,32 @@
+package chart
+
+// ConcatSeries is a special type of series that concatenates its `InnerSeries`.
+type ConcatSeries []Series
+
+// Len returns the length of the concatenated set of series.
+func (cs ConcatSeries) Len() int {
+	total := 0
+	for _, s := range cs {
+		if typed, isValueProvider := s.(ValueProvider); isValueProvider {
+			total += typed.Len()
+		}
+	}
+
+	return total
+}
+
+// GetValue returns the value at the (meta) index (i.e 0 => totalLen-1)
+func (cs ConcatSeries) GetValue(index int) (x, y float64) {
+	cursor := 0
+	for _, s := range cs {
+		if typed, isValueProvider := s.(ValueProvider); isValueProvider {
+			len := typed.Len()
+			if index < cursor+len {
+				x, y = typed.GetValue(index - cursor) //FENCEPOSTS.
+				return
+			}
+			cursor += typed.Len()
+		}
+	}
+	return
+}