Highlight correct point when series has null values

public/app/plugins/panel/graph/graph_tooltip.js

@@ -22,10 +22,13 @@ function ($) {
       var len = series.datapoints.points.length;
       for (var j = initial; j < len; j += ps) {
         if (series.datapoints.points[j] > posX) {
-          return Math.max(j - ps,  0)/ps;
+          break;
         }
       }
-      return j/ps - 1;
+      // Special case of a non stepped line, highlight the very last point just before a null point
+      while(!series.lines.steps && series.datapoints.points[initial] != null && j>0 && series.datapoints.points[j-ps] == null)
+        j-=ps;
+      return Math.max(j - ps,  0)/ps;
     };
 
     this.findHoverIndexFromData = function(posX, series) {