Add question mark icon for tooltip

panels/histogram/editor.html

@@ -62,7 +62,7 @@
   <h5>Tooltip Settings</h5>
   <div class="row-fluid" style="margin-bottom:10px;">
     <div class="span3">
-      <label class="small" bs-tooltip="'How should the values in stacked charts to be calculated?'">Stacked Values</label>
+      <label class="small">Stacked Values <i class="icon-question-sign" bs-tooltip="'How should the values in stacked charts to be calculated?'"></i></label>
       <select class="input-medium" ng-model="panel.tooltip.value_type" ng-options="f for f in ['cumulative','individual']"></select>
     </div>
   </div>
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