Some minor issues in nicholsplot

The Nyquist frequency is half the sampling frequency and not equal to it.

If frequency vector is specified it should perhaps not be truncated at the Nyquist frequency, but rather a point should mark it on the Nichols curve.

The arguments to the plot function are very short, val, sat and pInc doesn't say much about what they are used for. They are only used once in the code, so a longer, more descriptive name would make the code more self documenting for example:
gain_line_hue
gain_line_saturation
or something similar.

I find the unicode characters for real and imaginary parts are hard to read (subjective), would prefer just write Re and Im.

Start at the nicholsplot entry point and inspect how the Nyquist frequency, supplied frequency vectors, plot argument names, and real/imaginary labels are handled. Verify the expected Nyquist marker and frequency range, then check the resulting plot output and related tests if available; done means the reported plotting and readability issues are addressed.