code: def setup_params(self, data):
          
          # Variable for which to do the stacking
          if 'ymax' in data:
              if any((data['ymin'] != 0) & (data['ymax'] != 0)):
                  warn("Stacking not well defined when not "
                       "anchored on the axis.", PlotnineWarning)
              var = 'ymax'
          elif 'y' in data:
              var = 'y'
          else:
              warn("Stacking requires either ymin & ymax or y "
                   "aesthetics. Maybe you want position = 'identity'?",
                   PlotnineWarning)
              var = None
  
          params = self.params.copy()
          params['var'] = var
          params['fill'] = self.fill
          return params
docs: Verify, modify & return a copy of the params.