Fixed mistake in code from pull request #50

- The faster toFixed alternative now returns a string, as tickFormatter
is expected to do

jquery.flot.js

@@ -1220,7 +1220,7 @@
 
                 axis.tickFormatter = function (v, axis) {
                     var factor = Math.pow(10, axis.tickDecimals);
-                    return Math.round(v * factor) / factor;
+                    return "" + Math.round(v * factor) / factor;
                 };
             }