code: def observed_vis(self, context):
          
          lrow, urow = MS.row_extents(context)
  
          data = self._manager.ordered_main_table.getcol(
              self._vis_column, startrow=lrow, nrow=urow-lrow)
  
          return data.reshape(context.shape).astype(context.dtype)
docs: Observed visibility data source