Feature Request: Nichols plot with the closed-loop response on the rectilinear grid

[scottshambaugh]

In my experience in industry, Nichols plots have always been shown with the closed loop gain and phase response as the X-Y coordinates (essentially "unwrapping" the Nichols grid). This makes it easier to read off the closed-loop margins as opposed to squinting and trying to figure out how far between curved contour lines you are.

Instead of passing in the open-loop system Gol, the user can do this fairly easily right now by plotting the Nichols for their closed loop system Gcl = Gol/(1+Gol) and turning off the grid, so if people think this feature would just add confusion feel free to ignore.

[roryyorke]

As you noted, it's fairly easy to do something like ct.nichols(ct.feedback(g,1)), so I'm not sure we need to add this.

I am curious to know why you're interested in the closed-loop phase -- I normally only care about Bode magnitude plots of closed-loop systems, so I can see frequency and closed-loop gain crossover.