def cepline(qindex, cepstrum, ax, sr=None):
    """Draw a 1-D cepstrum line on an axis."""
    if sr is not None:
        qaxis = qindex/sr
        xlabel = 'Quefrency (s)'
    else:
        qaxis = qindex
        xlabel = 'Quefrency (sample)'
    line, = ax.plot(qaxis, cepstrum)
    ax.axis('tight')
    ax.set_xlabel(xlabel)
    ax.set_ylabel('Amplitude')
    return line