code: def set_customjs_callback(self, js_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
docs: Generates a CustomJS callback by generating the required JS
          code and gathering all plotting handles and installs it on
          the requested callback handle.