docs: def validate_average_best_span():
      """Figure 2d? from Friedman."""
      N = 200
      num_trials = 400
      avg = numpy.zeros(N)
      for i in range(num_trials):
          x, y = smoother_friedman82.build_sample_smoother_problem_friedman82(N=N)
          my_smoother = smoother.perform_smooth(
              x, y, smoother_cls=supersmoother.SuperSmoother
          )
          avg += my_smoother._smoothed_best_spans.smooth_result
          if not (i + 1) % 20:
              print(i + 1)
      avg /= num_trials
      plt.plot(my_smoother.x, avg, '.', label='Average JCV')
      finish_plot()