NG-Spice / Macspice - plot source current?
Spice 3 derivatives often convert names in netlists to all lowercase, but the command interpreter is usually case sensitive. This confuses many new users; try 'plot i(vin)'. The mixed-case syntax you describe, however, does work okay in MacSpice. This example:

Stackexchange MacSpice Example
Vin 1 0 dc 0 sin(0.0 1.0 50)
r1 1 0 1.2K
.control
tran 0.1ms 20ms 0.0 0.5ms
plot i(Vin)
.endc
.end


Produces this plot: