docs: def diag(ax=None, linecolor='0.0', linestyle='--', **kwargs):
      """Plot the diagonal."""
      ax = get_ax(ax)
      xy_min = np.min((ax.get_xlim(), ax.get_ylim()))
      xy_max = np.max((ax.get_ylim(), ax.get_xlim()))
      return ax.plot([xy_min, xy_max], [xy_min, xy_max],
                     ls=linestyle,
                     c=linecolor,
                     **kwargs)