I used this syntax: v(@m.xm1.msky130_fd_pr__nfet_01v8[w]) If you give the following command to xschem:

xschem raw_query list

it will return the names of saved nodes. For some reason ngspice puts v(...) around the expression for W.

xschem [~/.xschem/xschem_library/xschem_sky130] xschem raw_query list
v(v-sweep)
@m.xm1.msky130_fd_pr__nfet_01v8[gm]
i(@m.xm1.msky130_fd_pr__nfet_01v8[id])
v(@m.xm1.msky130_fd_pr__nfet_01v8[w])
v(d)
v(g)
v(m.xm1.msky130_fd_pr__nfet_01v8#body)
v(m.xm1.msky130_fd_pr__nfet_01v8#dbody)
v(m.xm1.msky130_fd_pr__nfet_01v8#sbody)
i(v1)
i(v2)