Added graphite function changed, closes #1961

public/app/plugins/datasource/graphite/gfunc.js

@@ -327,6 +327,13 @@ function (_, $) {
     defaultParams: [100]
   });
 
+  addFuncDef({
+    name: "changed",
+    category: categories.Special,
+    params: [],
+    defaultParams: []
+  });
+
   addFuncDef({
     name: 'scale',
     category: categories.Transform,