Look, this is my file, in which I try to graph id vs vgs, id vs vds, but I had to do it separately in 2 .raw, since it doesn't work for me in one

one first comment: do a

unset appendwrite

between 1st and 2nd while loops, otherwise the nmos_sweep_2.raw will keep growing. The first write must be done without set appendwrite so the old file is deleted.