[Plot] Get/set from overlay switcher

Get/set mode from the plot mode switcher; only show
the switcher if there is more than one mode available
(which should only occur when there is more than one
telemetry object in the plot.) WTD-625.

platform/features/plot/res/templates/plot.html

@@ -85,6 +85,7 @@
             </a>
 
             <div class="menu-element btn icon-btn very-subtle btn-menu dropdown click-invoke"
+                 ng-if="plot.getModeOptions().length > 1"
                  ng-controller="ClickAwayController as toggle">
 
                 <span ng-click="toggle.toggle()">