Heatmap z event data should derive from _z calc attribute for consistency, closes #1141

Author: cpsievert

inst/htmlwidgets/plotly.js

@@ -244,12 +244,13 @@ HTMLWidgets.widget({
         var trace = gd.data[pt.curveNumber];
 
         // Add other attributes here, if desired
+        var z = trace.type === "heatmap" ? "_z" : "z";
         if (!trace._isSimpleKey) {
-          var attrsToAttach = ["key", "z"];
+          var attrsToAttach = ["key", z];
         } else {
           // simple keys fire the whole key
           obj.key = trace.key;
-          var attrsToAttach = ["z"];
+          var attrsToAttach = [z];
         }
 
         for (var i = 0; i < attrsToAttach.length; i++) {