Issue with Plot of Fmax of MOS Transistor

Hello There,

I am trying to simulate  Fmax of MOS transistor.  for this I am following Art's  guideline  - from post "Measuring Transistor fmax"

When I plot Gumx from ADE direct plot. I see kind of weird graph. Could you please let me know what is problem with it ?

Attached : Schematic & Gumx graph