@staticmethod
def _mtr_xy(calc_x, calc_y, mtr_of='e00200p', tax_type='combined'):
    """
        Computes marginal tax rates for Calculator objects calc_x and calc_y
        for specified mtr_of income type and specified tax_type.
        """
    _, iitax_x, combined_x = calc_x.mtr(mtr_of, wrt_full_compensation=True)
    _, iitax_y, combined_y = calc_y.mtr(mtr_of, wrt_full_compensation=True)
    if tax_type == 'combined':
        return combined_x, combined_y
    elif tax_type == 'iitax':
        return iitax_x, iitax_y
    else:
        raise ValueError('tax_type must be "combined" or "iitax"')
