Fit RLM

def rlm(data, xseq, **params):
    """
    Fit RLM
    """
    X = sm.add_constant(data['x'])
    Xseq = sm.add_constant(xseq)

    init_kwargs, fit_kwargs = separate_method_kwargs(
        params['method_args'], sm.RLM, sm.RLM.fit)
    model = sm.RLM(data['y'], X, **init_kwargs)
    results = model.fit(**fit_kwargs)

    data = pd.DataFrame({'x': xseq})
    data['y'] = results.predict(Xseq)

    if params['se']:
        warnings.warn("Confidence intervals are not yet implemented"
                      "for RLM smoothing.", PlotnineWarning)

    return data