[Plot] Begin refactoring for stacked plots

Begin refactoring Plot view to support stacked plots;
separate out behavior which needs to occur per-plot
from area which should occur per-view, and add a
handler for the Overlay mode. WTD-625.

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

@@ -0,0 +1,60 @@
+/*global define*/
+
+define(
+    ["../SubPlot", "../elements/PlotPalette", "../elements/PlotPanZoomStack"],
+    function (SubPlot, PlotPalette, PlotPanZoomStack) {
+        "use strict";
+
+        function PlotOverlayMode(telemetryObjects) {
+            var domainOffset,
+                panZoomStack = new PlotPanZoomStack([], []),
+                subplot = new SubPlot(telemetryObjects, panZoomStack),
+                subplots = [ subplot ];
+
+            function plotTelemetry(prepared) {
+                 // Fit to the boundaries of the data, but don't
+                // override any user-initiated pan-zoom changes.
+                panZoomStack.setBasePanZoom(
+                    prepared.getOrigin(),
+                    prepared.getDimensions()
+                );
+
+                // Track the domain offset, used to bias domain values
+                // to minimize loss of precision when converted to 32-bit
+                // floating point values for display.
+                domainOffset = prepared.getDomainOffset();
+
+                // Draw the buffers. Select color by index.
+                subplot.getDrawingObject().lines = prepared.getBuffers().map(function (buf, i) {
+                    return {
+                        buffer: buf,
+                        color: PlotPalette.getFloatColor(i),
+                        points: buf.length / 2
+                    };
+                });
+
+                subplot.update();
+            }
+
+            return {
+                plotTelemetry: plotTelemetry,
+                getSubPlots: function () {
+                    return subplots;
+                },
+                isZoomed: function () {
+                    return panZoomStack.getDepth() > 1;
+                },
+                stepBackPanZoom: function () {
+                    panZoomStack.pop();
+                    subplot.update();
+                },
+                unzoom: function () {
+                    panZoomStack.clearPanZoom();
+                    subplot.update();
+                }
+            };
+        }
+
+        return PlotOverlayMode;
+    }
+);