fixes to InMemory setup

raster_renderer.go

@@ -23,9 +23,17 @@ func PNG(width, height int) (Renderer, error) {
 	return nil, err
 }
 
-// InMemory returns an in-memory raster renderer that calls cb() with the
+// InMemory returns an in-memory raster RenderProvider that calls cb() with the
 // rendered image upon completion.
-func InMemory(width, height int, cb func(image.Image)) (Renderer, error) {
+//
+// Usage suggestion:
+//
+//     ...
+//     chart.Render(chart.InMemory(mycollector.SetImage))
+//     ...
+//
+func InMemory(cb func(image.Image)) RendererProvider {
+	return func(width, height int) (Renderer, error) {
 		i := image.NewRGBA(image.Rect(0, 0, width, height))
 		gc, err := drawing.NewRasterGraphicContext(i)
 		if err == nil {
@@ -38,6 +46,7 @@ func InMemory(width, height int, cb func(image.Image)) (Renderer, error) {
 			}, nil
 		}
 		return nil, err
+	}
 }
 
 // rasterRenderer renders chart commands to a bitmap.