How to plot gm in ADS?

Hi friends,
I want to plot Nmos's gm and it's differential coefficient.I got a photo,but i do not how to get them:i do not know what is diffX(I only can find diff() function ) at X-axial and how to plot Y-axial(which equation?).
Anyone can help me?

Thanks,

Hi,

If you have only the current and the voltage you need to make a sweep in VGS then only put gm=diff(IDS)
I can help you if you send me more info

thanks

Thanks,eleazar.I want to use ADS to express the function as attached.

and select the optimum bias to reduce nolinear.What i can get from the document(no english version) is as above.