While debugging some code I noticed that examples/pvtol-nested.py was producing a blank figure instead of a pole-zero map that was expected. I chased this down tto the fact that the sgrid() function calls matplotlib.pyplot.figure(). This is in contrast to all other plotting routines and the consequence is that you get a new set of axes. This causes the following code (from examples/pvtol-nested.py) not to work as expected:

figure(10); clf();
(P, Z) = pzmap(T, Plot=True)
print("Closed loop poles and zeros: ", P, Z)

# Gang of Four
figure(11); clf();
gangof4(Hi*Po, Co);

Figure 10, which should have the output of pzmap will be blank since pzmap calls figure() (through sgrid) and creates a new set of axis (figure #11). Figure #11 will then get erased (away goes the pole/zero map) and be replace by the Gang of 4.