@User the vsource_pwl xschem symbol is for hspice. The ngspice equivalent can be done with the '`bsource.sym'` symbol, with the attributes

VAR

set to

V

(voltage) and

FUNC

to

pwl(V(node), 0, 0, 1, 3,...)

See the ngspice manual '5.1 Bxxxx: Nonlinear dependent source (ASRC)'