Generates a CustomJS callback by generating the required JS
        code and gathering all plotting handles and installs it on
        the requested callback handle.

def set_customjs_callback(self, js_callback, handle):
        """
        Generates a CustomJS callback by generating the required JS
        code and gathering all plotting handles and installs it on
        the requested callback handle.
        """
        if self.on_events:
            for event in self.on_events:
                handle.js_on_event(event, js_callback)
        if self.on_changes:
            for change in self.on_changes:
                handle.js_on_change(change, js_callback)
        elif hasattr(handle, 'callback'):
            handle.callback = js_callback