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/scatterplot/scatterplot.js

@@ -16,10 +16,11 @@ import {
   createColorQuery,
 } from "../../util/stateManager/colorHelpers";
 import renderThrottle from "../../util/renderThrottle";
-
-const flagSelected = 1;
-const flagNaN = 2;
-const flagHighlight = 4;
+import {
+  flagBackground,
+  flagSelected,
+  flagHighlight,
+} from "../../util/glHelpers";
 
 function createProjectionTF(viewportWidth, viewportHeight) {
   /*
@@ -135,8 +136,9 @@ class Scatterplot extends React.PureComponent {
     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;
         }
       }
     }