code: def scroll_event(self, widget, event):
          
          x, y = event.x, event.y
          num_degrees = 0
          direction = 0
  
          # x, y = coordinates of mouse
          self.last_win_x, self.last_win_y = x, y
  
          # calculate number of degrees of scroll and direction of scroll
          # both floats in the 0-359.999 range
          # num_degrees =
          # direction =
          self.logger.debug("scroll deg=%f direction=%f" % (
              num_degrees, direction))
  
          data_x, data_y = self.check_cursor_location()
  
          return self.make_ui_callback('scroll', direction, num_degrees,
                                       data_x, data_y)
docs: Called when a mouse is turned in the widget (and maybe for
          finger scrolling in the trackpad).
          Adjust method signature as appropriate for callback.