remember what is the Laplace transforme of capacitor is something as 1/Cs with s=jw
Now if you use a logarithmique scale (as Bode plot) what do you get ?
-20 Log(w)-20Log(C) (So a draw in a X logarithmique scale).Nevertheless you need to polarize your PMOS Capacitor before to perform this AC simulation.
I hope it will help you on problem