Add diffSeries function support

src/app/services/graphite/gfunc.js

@@ -63,6 +63,11 @@ function (_) {
     category: categories.Combine,
   });
 
+  addFuncDef({
+    name: 'diffSeries',
+    category: categories.Combine,
+  });
+
   addFuncDef({
     name: 'averageSeries',
     shortName: 'avg',