Cache wikidata results

Because wikidata field potentially shown on hover now
and the results of this lookup will never change

modules/services/wikidata.js

@@ -1,15 +1,18 @@
 import { json as d3_json } from 'd3-request';
 
 import { utilQsString } from '../util';
-
 import { currentLocale } from '../util/locale';
 
 var endpoint = 'https://www.wikidata.org/w/api.php?';
+var _wikidataCache = {};
 
 export default {
 
     init: function() {},
-    reset: function() {},
+
+    reset: function() {
+        _wikidataCache = {};
+    },
 
 
     // Given a Wikipedia language and article title, return an array of
@@ -42,6 +45,10 @@ export default {
             callback('', {});
             return;
         }
+        if (_wikidataCache[qid]) {
+            callback('', _wikidataCache[qid]);
+            return
+        }
 
         var lang = currentLocale.replace(/-/g, '_');
 
@@ -58,6 +65,7 @@ export default {
             if (err || !data || data.error) {
                 callback('', {});
             } else {
+                _wikidataCache[qid] = data.entities[qid];
                 callback(qid, data.entities[qid] || {});
             }
         });