code: def plot_intrusion_curve(self, fig=None):
          r
          # Begin creating nicely formatted plot
          x, y = self.get_intrusion_data()
          if fig is None:
              fig = plt.figure()
          plt.semilogx(x, y, 'ko-')
          plt.ylabel('Invading Phase Saturation')
          plt.xlabel('Capillary Pressure')
          plt.grid(True)
          return fig
docs: r"""
          Plot the percolation curve as the invader volume or number fraction vs
          the applied capillary pressure.