Use kmeans to automatically group map data

See http://roadtolarissa.com/coloring-maps/ and
http://bl.ocks.org/thadk/37ff4c03358b414acd1e as to why this is a better
solution than quantize.

frontend/src/metabase/visualizations/components/ChoroplethMap.jsx

@@ -17,6 +17,7 @@ import LeafletChoropleth from "./LeafletChoropleth.jsx";
 import { computeMinimalBounds } from "metabase/visualizations/lib/mapping";
 
 import d3 from "d3";
+import ss from "simple-statistics";
 import _ from "underscore";
 
 // const HEAT_MAP_COLORS = [
@@ -162,15 +163,20 @@ export default class ChoroplethMap extends Component {
         }
 
         const valuesMap = {};
+        const domain = []
         for (const row of rows) {
             valuesMap[getRowKey(row)] = (valuesMap[getRowKey(row)] || 0) + getRowValue(row);
+            domain.push(getRowValue(row));
         }
 
-        var colorScale = d3.scale.quantize().domain(d3.extent(rows, getRowValue)).range(HEAT_MAP_COLORS);
+        const groups = ss.ckmeans(domain, HEAT_MAP_COLORS.length);
+
+        var colorScale = d3.scale.quantile().domain(groups.map((cluster) => cluster[0])).range(HEAT_MAP_COLORS);
 
         let legendColors = HEAT_MAP_COLORS.slice();
         let legendTitles = HEAT_MAP_COLORS.map((color, index) => {
-            let [min, max] = colorScale.invertExtent(color);
+            const min = groups[index][0];
+            const max = groups[index].slice(-1)[0];
             return index === HEAT_MAP_COLORS.length - 1 ?
                 formatNumber(min) + " +" :
                 formatNumber(min) + " - " + formatNumber(max)

package.json

@@ -62,6 +62,7 @@
     "reselect": "^2.0.1",
     "resize-observer-polyfill": "^1.3.2",
     "screenfull": "^3.0.0",
+    "simple-statistics": "^2.5.0",
     "stack-source-map": "^1.0.4",
     "tether": "^1.2.0",
     "underscore": "^1.8.3",

yarn.lock

@@ -7120,6 +7120,10 @@ signal-exit@^3.0.0:
   version "3.0.2"
   resolved "https://registry.yarnpkg.com/signal-exit/-/signal-exit-3.0.2.tgz#b5fdc08f1287ea1178628e415e25132b73646c6d"
 
+simple-statistics@^2.5.0:
+  version "2.5.0"
+  resolved "https://registry.yarnpkg.com/simple-statistics/-/simple-statistics-2.5.0.tgz#7cd77b04288402f17fe21fd2b63c5e1a51ef3f51"
+
 simple-swizzle@^0.2.2:
   version "0.2.2"
   resolved "https://registry.yarnpkg.com/simple-swizzle/-/simple-swizzle-0.2.2.tgz#a4da6b635ffcccca33f70d17cb92592de95e557a"