code: def filter_butter_plot_freqresponse(b, a, samplerate, numpoints=2000):
      # type: (Seq[float], Seq[float], int, int) -> None
      
      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()
docs: Plot the freq. response of the digital butterw. filter 
      defined by the coeffs. (b, a) at `samplerate`
  
      Returns --> nothing
  
      See also: filter_butter_coeffs