Handle 'group' graphite method

src/app/services/graphite/gfunc.js

@@ -86,6 +86,13 @@ function (_) {
     category: categories.Calculate,
   });
 
+  addFuncDef({
+    name: 'group',
+    params: optionalSeriesRefArgs,
+    defaultParams: ['#A', '#B'],
+    category: categories.Combine,
+  });
+
   addFuncDef({
     name: 'sumSeries',
     shortName: 'sum',