Use trace.uid rather than hanging on to the entire trace just for the lastPickResults

(cherry picked from commit a2efeb4)

Author: monfera

src/plots/gl2d/scene2d.js

@@ -483,13 +483,13 @@ proto.draw = function() {
 
             if(nextSelection && (
                 !this.lastPickResult ||
-                this.lastPickResult.trace !== nextSelection.trace ||
+                this.lastPickResult.traceUid !== nextSelection.trace.uid ||
                 this.lastPickResult.dataCoord[0] !== nextSelection.dataCoord[0] ||
                 this.lastPickResult.dataCoord[1] !== nextSelection.dataCoord[1])
             ) {
                 var selection = nextSelection;
                 this.lastPickResult = {
-                    trace: nextSelection.trace, // could we just retain/compare the trace uid?
+                    traceUid: nextSelection.trace ? nextSelection.trace.uid : null,
                     dataCoord: nextSelection.dataCoord.slice()
                 };
                 this.spikes.update({ center: result.dataCoord });