docs: def plotter_cls(self):
          """The plotter class"""
          ret = self._plotter_cls
          if ret is None:
              self._logger.debug('importing %s', self.module)
              mod = import_module(self.module)
              plotter = self.plotter_name
              if plotter not in vars(mod):
                  raise ImportError("Module %r does not have a %r plotter!" % (
                      mod, plotter))
              ret = self._plotter_cls = getattr(mod, plotter)
              _versions.update(get_versions(key=lambda s: s == self._plugin))
          return ret