code: def plot_radius(self):
          
          plot_rad = 0
          for group, nodelist in self.nodes.items():
              proposed_radius = len(nodelist) * self.scale
              if proposed_radius > plot_rad:
                  plot_rad = proposed_radius
          return plot_rad + self.internal_radius
docs: Computes the plot radius: maximum of length of each list of nodes.