code: def scroll_cb(self, viewer, direction, amt, data_x, data_y):
          
          bd = viewer.get_bindings()
          direction = bd.get_direction(direction)
          pan_x, pan_y = viewer.get_pan()[:2]
          qty = self._cmsep * amt * self.settings.get('cbar_pan_accel', 1.0)
          if direction == 'up':
              pan_y -= qty
          else:
              pan_y += qty
  
          pan_y = min(max(pan_y, 0), self._max_y)
  
          viewer.set_pan(pan_x, pan_y)
docs: Called when the user scrolls in the color bar viewer.
          Pan up or down to show additional bars.