@Ryan R I don't think there is a stacked waveform option in ngspice (which basically uses the gnuplot plotting functions). The trick is to shift the voltage values adding some fixed quantities as you shown. You can plot multiple simulations by referencing the plot name:

plot tran1.v1 tran2.v1 tran3.v1

. You can append multiple simulations at different corners into the same raw file by using

set appendwrite

after the 1st run. You an also try xschem's own waveform display.