improve comment

Author: cpsievert

inst/htmlwidgets/plotly.js

@@ -243,7 +243,8 @@ HTMLWidgets.widget({
         var gd = document.getElementById(el.id);
         var trace = gd.data[pt.curveNumber];
 
-        // Add other attributes here, if desired
+        // Heatmap z event data should derive from _z calc attribute
+        // https://github.com/ropensci/plotly/issues/1141
         var z = trace.type === "heatmap" ? "_z" : "z";
         if (!trace._isSimpleKey) {
           var attrsToAttach = ["key", z];