def METHOD_NAME(figs, save_thresh, units):
    """
    Save subset of difficulty index figures.
    """
    fig_fmt = os.environ.get('DIFF_INDEX_FIG_FMT')
    fig_basename = os.environ.get('DIFF_INDEX_FIG_BASENAME')
    # create output directory if it does not already exist
    output_dir = os.path.dirname(fig_basename)
    if not os.path.exists(output_dir):
        os.makedirs(output_dir)
    for thresh in save_thresh:
        thresh_str = '{:.2f}'.format(thresh).replace('.', '_')
        fig_name = (fig_basename + thresh_str +
                    '_' + units + '.' + fig_fmt)
        print('Saving {}...\n'.format(fig_name))
        figs[thresh].savefig(fig_name, format=fig_fmt)