small fix to png rendering

src/app/directives/grafanaGraph.js

@@ -62,7 +62,7 @@ function (angular, $, kbn, moment, _) {
           if (!setElementHeight()) { return; }
 
           if (_.isString(data)) {
-            render_panel_as_graphite_png();
+            render_panel_as_graphite_png(data);
             return;
           }
 
@@ -166,28 +166,29 @@ function (angular, $, kbn, moment, _) {
           addAxisLabels();
         }
 
-        function render_panel_as_graphite_png() {
-          data += '&width=' + elem.width();
-          data += '&height=' + elem.css('height').replace('px', '');
-          data += '&bgcolor=1f1f1f'; // @grayDarker & @kibanaPanelBackground
-          data += '&fgcolor=BBBFC2'; // @textColor & @grayLighter
-          data += scope.panel.stack ? '&areaMode=stacked' : '';
-          data += scope.panel.fill !== 0 ? ('&areaAlpha=' + (scope.panel.fill/10).toFixed(1)) : '';
-          data += scope.panel.linewidth !== 0 ? '&lineWidth=' + scope.panel.linewidth : '';
-          data += scope.panel.steppedLine ? '&lineMode=staircase' : '';
+        function render_panel_as_graphite_png(url) {
+          url += '&width=' + elem.width();
+          url += '&height=' + elem.css('height').replace('px', '');
+          url += '&bgcolor=1f1f1f'; // @grayDarker & @kibanaPanelBackground
+          url += '&fgcolor=BBBFC2'; // @textColor & @grayLighter
+          url += scope.panel.stack ? '&areaMode=stacked' : '';
+          url += scope.panel.fill !== 0 ? ('&areaAlpha=' + (scope.panel.fill/10).toFixed(1)) : '';
+          url += scope.panel.linewidth !== 0 ? '&lineWidth=' + scope.panel.linewidth : '';
 
           switch(scope.panel.nullPointMode) {
           case 'connected':
-            data += '&lineMode=connected';
+            url += '&lineMode=connected';
             break;
           case 'null':
             break; // graphite default lineMode
           case 'null as zero':
-            data += "&drawNullAsZero=true";
+            url += "&drawNullAsZero=true";
             break;
           }
 
-          elem.html('<img src="' + data + '"></img>');
+          url += scope.panel.steppedLine ? '&lineMode=staircase' : '';
+
+          elem.html('<img src="' + url + '"></img>');
         }