docs: def plot2d(self, color='default', alpha=1, ret=True):
          """
          Generates a 2D plot for the z=0 Polygon projection.
          
          :param color: Polygon color.
          :type color: matplotlib color
          :param alpha: Opacity.
          :type alpha: float
          :param ret: If True, returns the figure. It can be used to add 
              more elements to the plot or to modify it.
          :type ret: bool
          :returns: None, axes
          :rtype: None, matplotlib axes
          """
          import matplotlib.pyplot as plt
          import matplotlib.patches as patches
          
          path = self.get_path()
          domain = self.get_domain()[:, :2]
  
          if color is 'default': color = 'b'
  
          # Plot
          fig = plt.figure()
          ax = fig.add_subplot(111)
          ax.add_patch(patches.PathPatch(path, facecolor=color, lw=1, 
                                         edgecolor='k', alpha=alpha))
          ax.set_xlim(domain[0,0],domain[1,0])
          ax.set_ylim(domain[0,1], domain[1,1])
          if ret: return ax