docs: def plot(self, fname=None):
          """
          Plot the input data and resulting smooth.
  
          Parameters
          ----------
          fname : str, optional
              name of file to produce. If none, will show interactively.
          """
          plt.figure()
          xy = sorted(zip(self.x, self.smooth_result))
          x, y = zip(*xy)
          plt.plot(x, y, '-')
          plt.plot(self.x, self.y, '.')
          if fname:
              plt.savefig(fname)
          else:
              plt.show()
          plt.close()