docs: def plot_input(ace_model, fname='ace_input.png'):
      """Plot the transforms."""
      if not plt:
          raise ImportError('Cannot plot without the matplotlib package')
      plt.rcParams.update({'font.size': 8})
      plt.figure()
      num_cols = len(ace_model.x) / 2 + 1
      for i in range(len(ace_model.x)):
          plt.subplot(num_cols, 2, i + 1)
          plt.plot(ace_model.x[i], ace_model.y, '.')
          plt.xlabel('x{0}'.format(i))
          plt.ylabel('y')
  
      plt.tight_layout()
  
      if fname:
          plt.savefig(fname)
      else:
          plt.show()