Using calculator. U can calculate Vg-Vs, then click MP calculator button and select transistor on schematic window. After that choose vth0 model parameter and subtract that from Vg-Vs. You will get expression Vs-Vg-Vth. Just plot it.
Not sure how your method work, but i used the lshift function to shift the x-axis (Vgs) by -Vth using OP function. Now, if I wanted to times by a constant then I am not sure how to do it. Any ideas? Maybe interchange y and x axis but not sure how to it either.