Today I used the following code to produce the nyquist plot of an arbitrary transfer function:

import numpy as npfrom control.matlab import *import matplotlib.pyplot as plta0 = 1.; a1 = 10.; a2 = 31.; a3 = 30b0 = 1.num = np.array([b0])den = np.array([a3, a2, a1, a0])system_tf = tf(num,den)ctrl_tf = tf([5, 1],[5, 0])G0 = ctrl_tf*system_tfGw = feedback(G0,1)nyquist(Gw)plt.show()

and I got this strange discontinuous Nyquist plot:

the bode plot looks correct and I found that both plots use different functions to evaluate the frequency response. So I think there might be a bug in the implementation of the nyquist plot.

Here is the expected Nyquist plot that I could obtain from Matlab.