def METHOD_NAME(mu_fig, sigma_fig):
    """
    Save ensemble mean and spread figures.
    """
    fig_fmt = os.environ.get('DIFF_INDEX_FIG_FMT')
    fig_basename = os.environ.get('DIFF_INDEX_FIG_BASENAME')
    mu_name = fig_basename + 'mean.' + fig_fmt
    print('Saving {}...\n'.format(mu_name))
    mu_fig.savefig(mu_name, format=fig_fmt)
    sigma_name = fig_basename + 'std.' + fig_fmt
    print('Saving {}...\n'.format(sigma_name))
    sigma_fig.savefig(sigma_name, format=fig_fmt)