diff --git a/image_writer.go b/image_writer.go
index b4ddb7a..e9cfd7a 100644
--- a/image_writer.go
+++ b/image_writer.go
@@ -7,6 +7,7 @@ import (
 	"image/png"
 )
 
+// RGBACollector is a render target for a chart.
 type RGBACollector interface {
 	SetRGBA(i *image.RGBA)
 }