[Plot] Add skeleton specs for stacked plots

Add skeleton specs for classes introduced to support
stacked plot mode, WTD-625.

platform/features/plot/test/elements/PlotPanZoomStackGroupSpec.js

@@ -0,0 +1,14 @@
+/*global define,Promise,describe,it,expect,beforeEach,waitsFor,jasmine*/
+
+/**
+ * MergeModelsSpec. Created by vwoeltje on 11/6/14.
+ */
+define(
+    ["../../src/elements/PlotPanZoomStackGroup"],
+    function (PlotController) {
+        "use strict";
+
+        describe("A plot pan-zoom stack group", function () {
+        });
+    }
+);