alpha blending fix in scatterplots (#2033)

* explicitly specify alpha blending; rename NaN flag to accurately describe function

* fix incorret dest alpha blending function

client/src/components/graph/graph.js

@@ -22,6 +22,12 @@ import CentroidLabels from "./overlays/centroidLabels";
 import actions from "../../actions";
 import renderThrottle from "../../util/renderThrottle";
 
+import {
+  flagBackground,
+  flagSelected,
+  flagHighlight,
+} from "../../util/glHelpers";
+
 /*
 Simple 2D transforms control all point painting.  There are three:
   * model - convert from underlying per-point coordinate to a layout.
@@ -62,10 +68,6 @@ function createModelTF() {
   return m;
 }
 
-const flagSelected = 1;
-const flagNaN = 2;
-const flagHighlight = 4;
-
 @connect((state) => ({
   annoMatrix: state.annoMatrix,
   crossfilter: state.obsCrossfilter,
@@ -159,8 +161,9 @@ class Graph extends React.Component {
     const flags = new Float32Array(nObs);
     if (colorByData) {
       for (let i = 0, len = flags.length; i < len; i += 1) {
-        if (!Number.isFinite(colorByData[i])) {
-          flags[i] = flagNaN;
+        const val = colorByData[i];
+        if (typeof val === "number" && !Number.isFinite(val)) {
+          flags[i] = flagBackground;
         }
       }
     }