Fixed the refineString bug

js/tracer_manager.js

@@ -193,7 +193,7 @@ var TracerUtil = {
       return $.isNumeric(item) ? this.refineNumber(item) : this.refineString(item);  
     },
     refineString: function (string) {
-        return string === Infinity ? '∞' : string;
+        return string === '' ? ' ' : string;
     },
     refineNumber: function (number) {
         return number === Infinity ? '∞' : number;