refactor stat_graph_contributors to es6 module syntax

app/assets/javascripts/graphs/graphs_bundle.js

-require('./stat_graph_contributors');
+import ContributorsStatGraph from './stat_graph_contributors';
+
+// export to global scope
+window.ContributorsStatGraph = ContributorsStatGraph;

app/assets/javascripts/graphs/stat_graph_contributors.js

-/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, one-var, camelcase, one-var-declaration-per-line, quotes, no-param-reassign, quote-props, comma-dangle, prefer-template, max-len, no-return-assign */
+/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, one-var, camelcase, one-var-declaration-per-line, quotes, no-param-reassign, quote-props, comma-dangle, prefer-template, max-len, no-return-assign, no-shadow */
 
+import d3 from 'd3';
 import { ContributorsGraph, ContributorsAuthorGraph, ContributorsMasterGraph } from './stat_graph_contributors_graph';
 import ContributorsStatGraphUtil from './stat_graph_contributors_util';
 
-/* global d3 */
-
-window.d3 = require('d3');
-
-(function() {
-  this.ContributorsStatGraph = (function() {
+export default (function() {
   function ContributorsStatGraph() {}
 
   ContributorsStatGraph.prototype.init = function(log) {
@@ -112,5 +108,4 @@ window.d3 = require('d3');
   };
 
   return ContributorsStatGraph;
-  })();
-}).call(window);
+})();