def _get_time_axis(time_list, units='hours'):
    """Convert time to sequential format and convert time units."""
    time_axis = []
    for i in range(len(time_list)):
        time_sum = sum(time_list[:i])
        if units == 'seconds':
            pass
        elif units == 'minutes':
            time_sum /= 60.0
        elif units == 'hours':
            time_sum /= 3600.0
        time_axis.append(time_sum)
    return time_axis