try sending hoverdata on double click:

- double click on a data pt now (1) fires a click event,
  (2) fires a double click event and (3) fires another click event,
  with the same event data
- double clicking not on a data pt fires a double click event
  with null event data

Author: etpinard

src/plots/cartesian/graph_interact.js

@@ -1821,7 +1821,8 @@ function dragBox(gd, plotinfo, x, y, w, h, ns, ew) {
             }
         }
 
-        gd.emit('plotly_doubleclick', null);
+        var dblClickData = gd._hoverdata ? {points: gd._hoverdata} : null;
+        gd.emit('plotly_doubleclick', dblClickData);
         Plotly.relayout(gd, attrs);
     }