def get_channels(ifo, plottype):
    """Get a list of channels to plot for a given IFO. Plot Type must be
    either 'irigb' or 'duotone'."""
    if plottype == "irigb":
        return ['{}:CAL-PCAL{}_IRIGB_OUT_DQ'.format(ifo, arm)
                for arm in ['X', 'Y']]
    elif plottype == "duotone":
        return ['{}:CAL-PCAL{}_FPGA_DTONE_IN1_DQ'.format(ifo, arm)
                for arm in ['X', 'Y']]
    else:
        raise ValueError("Must specify 'irigb' or 'duotone' for plottype.")