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

import numpy as np
from control.matlab import *
import matplotlib.pyplot as plt

a0 = 1.; a1 = 10.; a2 = 31.; a3 = 30
b0 = 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_tf
Gw = 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.