[Plot] Initially implement synced pan/zoom stack

Initially implement a synchronized pan/zoom stack,
which maintains the same domain axis bounds for all
subplots in a stacked plot view, but allows each to
have independent range axes. WTD-625.

platform/features/plot/src/elements/PlotPanZoomStackGroup.js

@@ -0,0 +1,79 @@
+/*global define*/
+
+define(
+    ['./PlotPanZoomStack'],
+    function (PlotPanZoomStack) {
+        "use strict";
+
+        function PlotPanZoomStackGroup(count) {
+            var stacks = [],
+                decoratedStacks = [],
+                i;
+
+            function pushPanZoom(origin, dimensions, index) {
+                stacks.forEach(function (stack, i) {
+                    if (i === index) {
+                        stack.pushPanZoom(origin, dimensions);
+                    } else {
+                        stack.pushPanZoom(
+                            [ origin[0], stack.getOrigin[1] ],
+                            [ dimensions[0], stack.getDimensions[1] ]
+                        );
+                    }
+                });
+            }
+
+            function setBasePanZoom(origin, dimensions, index) {
+                stacks.forEach(function (stack, i) {
+                    stack.setBasePanZoom(origin, dimensions);
+                });
+            }
+
+
+            function popPanZoom() {
+                stacks.forEach(function (stack) {
+                    stack.popPanZoom();
+                });
+            }
+
+            function clearPanZoom() {
+                stacks.forEach(function (stack) {
+                    stack.popPanZoom();
+                });
+            }
+
+            function decorateStack(stack, index) {
+                var result = Object.create(stack);
+
+                result.pushPanZoom = function (origin, dimensions) {
+                    pushPanZoom(origin, dimensions, index);
+                };
+                result.setBasePanZoom = function (origin, dimensions) {
+
+                };
+                result.popPanZoom = popPanZoom;
+                result.clearPanZoom = clearPanZoom;
+            }
+
+            for (i = 0; i < count; i += 1) {
+                stacks.push(new PlotPanZoomStack([], []));
+            }
+            decoratedStacks = stacks.map(decorateStack);
+
+            return {
+                popPanZoom: popPanZoom,
+                clearPanZoom: clearPanZoom,
+                getDepth: function () {
+                    return stacks.length > 0 ?
+                            stacks[0].getDepth() : 0;
+                },
+                getPanZoomStack: function (index) {
+                    return decoratedStacks[index];
+                }
+            };
+
+        }
+
+        return PlotPanZoomStackGroup;
+    }
+);

platform/features/plot/src/modes/PlotOverlayMode.js

@@ -45,7 +45,7 @@ define(
                     return panZoomStack.getDepth() > 1;
                 },
                 stepBackPanZoom: function () {
-                    panZoomStack.pop();
+                    panZoomStack.popPanZoom();
                     subplot.update();
                 },
                 unzoom: function () {