From PR #684: If I use the following code (from the now skipped unit test in `control/tests/rootlocus_test.py`: ``` syscont = ct.TransferFunction(1,[1, 3, 0]) ct.rootlocus_pid_designer(syscont, input_signal='r', Kp0=0.01, derivative_in_feedback_path=True) ``` then I get the following plot: 
