[Plot] Organize plot elements

Place elements of the PlotController into a separate
directory inside of the source folder for the plot
plug-in, in preparation for review and integration.
Part of ongoing plot transition to Angular, WTD-533.

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

@@ -0,0 +1,35 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        function PlotAxis(axisType, metadatas, defaultValue) {
+            var keys = {},
+                options = [];
+
+            function buildOptionsForMetadata(m) {
+                (m[axisType] || []).forEach(function (option) {
+                    if (!keys[option.key]) {
+                        keys[option.key] = true;
+                        options.push(option);
+                    }
+                });
+            }
+
+            (metadatas || []).forEach(buildOptionsForMetadata);
+
+            // Plot axis will be used directly from the Angular
+            // template, so expose properties directly to facilitate
+            // two-way data binding (for drop-down menus)
+            return {
+                options: options,
+                active: options[0] || defaultValue
+            };
+        }
+
+        return PlotAxis;
+
+    }
+);