code: def _score_clusters(self, X, y=None):
          
          stype = self.scoring.lower() # scoring method name
  
          if stype == "membership":
              return np.bincount(self.estimator.labels_)
  
          raise YellowbrickValueError("unknown scoring method '{}'".format(stype))
docs: Determines the "scores" of the cluster, the metric that determines the
          size of the cluster visualized on the visualization.