docs: def plot_grid(step):
      """Plot cell position and thickness.
  
      The figure is call grid_N.pdf where N is replace by the step index.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
      """
      rad = get_rprof(step, 'r')[0]
      drad = get_rprof(step, 'dr')[0]
      _, unit = step.sdat.scale(1, 'm')
      if unit:
          unit = ' ({})'.format(unit)
      fig, (ax1, ax2) = plt.subplots(2, sharex=True)
      ax1.plot(rad, '-ko')
      ax1.set_ylabel('$r$' + unit)
      ax2.plot(drad, '-ko')
      ax2.set_ylabel('$dr$' + unit)
      ax2.set_xlim([-0.5, len(rad) - 0.5])
      ax2.set_xlabel('Cell number')
      misc.saveplot(fig, 'grid', step.istep)