diff --git a/src/components/graph/drawPointsRegl.js b/src/components/graph/drawPointsRegl.js
index 50a1e826..7448bc41 100644
--- a/src/components/graph/drawPointsRegl.js
+++ b/src/components/graph/drawPointsRegl.js
@@ -1,5 +1,7 @@
 const mat4 = require('gl-mat4')
 
+// opacity: https://github.com/spacetx/starfish/blob/master/viz/draw/regions.js
+
 export default function (regl) {
   return regl({
     vert: `