Trigger an overlay refresh when drawing the plot.

Resolves #1030.  The performance impact should be low, and is the
correct thing to do in just about all cases where a plot is being
redrawn.

jquery.flot.js

@@ -1758,6 +1758,11 @@ Licensed under the MIT license.
             }
 
             surface.render();
+
+            // A draw implies that either the axes or data have changed, so we
+            // should probably update the overlay highlights as well.
+
+            triggerRedrawOverlay();
         }
 
         function extractRange(ranges, coord) {