Plot the freq. response of the digital butterw. filter 
    defined by the coeffs. (b, a) at `samplerate`

    Returns --> nothing

    See also: filter_butter_coeffs

def filter_butter_plot_freqresponse(b, a, samplerate, numpoints=2000):
    # type: (Seq[float], Seq[float], int, int) -> None
    """
    Plot the freq. response of the digital butterw. filter 
    defined by the coeffs. (b, a) at `samplerate`

    Returns --> nothing

    See also: filter_butter_coeffs
    """
    import matplotlib.pyplot as plt
    plt.figure(1)
    plt.clf()
    w, h = signal.freqz(b, a, worN=numpoints)
    plt.semilogx((samplerate*0.5/np.pi) * w, np.abs(h))
    plt.xlabel('Freq (Hz)')
    plt.ylabel('Gain')
    plt.grid(True)
    plt.show()