Allow users to specify label->color mapping (#3879)

Users can define `label_colors` in a dashboard's JSON metadata that
enforces a label to color mapping.

This also makes the function that maps labels to colors case insensitive.

(cherry picked from commit a82bb588f4)

superset/assets/javascripts/modules/colors.js

@@ -103,17 +103,36 @@ export const spectrums = {
   ],
 };
 
+/**
+ * Get a color from a scheme specific palette (scheme)
+ * The function cycles through the palette while memoizing labels
+ * association to colors. If the function is called twice with the
+ * same string, it will return the same color.
+ *
+ * @param {string} s - The label for which we want to get a color
+ * @param {string} scheme - The palette name, or "scheme"
+ * @param {string} forcedColor - A color that the caller wants to
+    forcibly associate to a label.
+ */
 export const getColorFromScheme = (function () {
-  // Color factory
   const seen = {};
-  return function (s, scheme) {
+  const forcedColors = {};
+  return function (s, scheme, forcedColor) {
     if (!s) {
       return;
     }
     const selectedScheme = scheme ? ALL_COLOR_SCHEMES[scheme] : ALL_COLOR_SCHEMES.bnbColors;
-    let stringifyS = String(s);
+    let stringifyS = String(s).toLowerCase();
     // next line is for superset series that should have the same color
     stringifyS = stringifyS.replace('---', '');
+
+    if (forcedColor && !forcedColors[stringifyS]) {
+      forcedColors[stringifyS] = forcedColor;
+    }
+    if (forcedColors[stringifyS]) {
+      return forcedColors[stringifyS];
+    }
+
     if (seen[selectedScheme] === undefined) {
       seen[selectedScheme] = {};
     }