fix rickshaw memory leaks in dashboards

replacing the DOM element that rickshaw attached to is not enough to get rid of the lingering event listeners which keep the SVG DOM elements alive
having one more level of DOM element fixes the issue and allows the garbage collector to do its work

fixes issue #782

Author: kroepke

public/javascripts/dashboards/widgets/field_chart.js

@@ -58,12 +58,14 @@ function updateWidget_field_chart(widget, data) {
         return;
     }
 
-    graphElem.html("");
+    // we need to replace the entire element that rickshaw touches, otherwise
+    // it will leak event listeners and tons of DOM elements
+    graphElem.html('<div class="graph_chart">');
 
     var renderer = graphElem.attr("data-config-renderer");
 
     var graph = new Rickshaw.Graph( {
-        element: graphElem.get()[0],
+        element: $('.graph_chart', graphElem)[0],
         width: 800,
         height: 70,
         interpolation: graphElem.attr("data-config-interpolation"),

public/javascripts/dashboards/widgets/search_result_chart.js

@@ -34,10 +34,12 @@ function updateWidget_search_result_chart(widget, data) {
         return;
     }
 
-    graphElem.html("");
+    // we need to replace the entire element that rickshaw touches, otherwise
+    // it will leak event listeners and tons of DOM elements
+    graphElem.html('<div class="graph_chart">');
 
     var graph = new Rickshaw.Graph( {
-        element: graphElem.get()[0],
+        element: $('.graph_chart', graphElem)[0],
         width: 800,
         height: 70,
         renderer: "bar",