def getSpectroscopicParmLabel(expt_type):
    """
    Returns the label for the spectroscopic parameter in the plot group. 
    
    Parameters
    ----------
    expt_type : str
        Type of the experiment - found in the parms.txt file
    
    Returns
    -------
    str
        label for the spectroscopic parameter axis in the plot
    """
    if expt_type in ['DC modulation mode', 'current mode']:
        return 'DC Bias'
    elif expt_type == 'AC modulation mode with time reversal':
        return 'AC amplitude'
    return 'User Defined'
