Removed unnecessary zIndex

src/graph/Graph.js

@@ -737,7 +737,6 @@ Graph.prototype._create = function () {
   this.frame.className = 'graph-frame';
   this.frame.style.position = 'relative';
   this.frame.style.overflow = 'hidden';
-  this.frame.style.zIndex = "1";
 
   // create the graph canvas (HTML canvas element)
   this.frame.canvas = document.createElement( 'canvas' );