code: def _add_item(self, cls, *args, **kwargs):
          
          box_index = kwargs.pop('box_index', self._default_box_index)
  
          data = cls.validate(*args, **kwargs)
          n = cls.vertex_count(**data)
  
          if not isinstance(box_index, np.ndarray):
              k = len(self._default_box_index)
              box_index = _get_array(box_index, (n, k))
          data['box_index'] = box_index
  
          if cls not in self._items:
              self._items[cls] = []
          self._items[cls].append(data)
          return data
docs: Add a plot item.