[Plot] Fix bug for panels

Fix bug wherein number of points-to-plot is misreported
with multiple traces in a plot; clean up on modifications
to plot to self-cache streaming telemetry, to remove
other cache, WTD-751.

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

@@ -194,7 +194,7 @@ define(
                  * @returns {number} the number of points in this buffer
                  */
                 getLength: function (index) {
-                    return lengthArray;
+                    return lengthArray[index] || 0;
                 },
                 /**
                  * Update with latest data.