Fail-safe when recomputing hover for click

Author: rreusser

src/plots/cartesian/graph_interact.js

@@ -1529,9 +1529,13 @@ function hoverChanged(gd, evt, oldhoverdata) {
 fx.click = function(gd, evt, subplot) {
     var annotationsDone = Registry.getComponentMethod('annotations', 'onClick')(gd, gd._hoverdata);
 
-    // The true flag at the end causes it to re-run the hover computation to figure out *which*
-    // point is being clicked. Without this, clicking is somewhat unreliable.
-    fx.hover(gd, evt, subplot, true);
+    // fallback to fail-safe in case the plot type's hover method doesn't pass the subplot.
+    // Ternary, for example, didn't, but it was caught because tested.
+    if (subplot !== undefined) {
+        // The true flag at the end causes it to re-run the hover computation to figure out *which*
+        // point is being clicked. Without this, clicking is somewhat unreliable.
+        fx.hover(gd, evt, subplot, true);
+    }
 
     function emitClick() { gd.emit('plotly_click', {points: gd._hoverdata, event: evt}); }