any info about parametric dc sweep ? for ex: want to plot gm/id vs vgs of a device and vgs is parameter not a voltage source I tried let and alterparam but ngspice gives two errors: 1- vector vgs is not available or has zero length 2- wrong format in alterparam line: skipping that line is there a simpler way to do this sweep ? or what's wrong here ? Thanks all! @Tim Edwards @Mitch Bailey

The best option is to run a DC sweep simulation. something like .dc VG 0 1.8 1m. Then you should plot the derivative of the current versus the current.

.control
  dc vtest4 0 1.8 1m
  let id = i(vtest5)
  let gm = deriv(id)
  plot gm vs id
.endc