you can also place a voltage source with voltage 0, then plot i(Vn) i don't think ngspice calculates currents unless you tell it to by using an ammeter or voltage source

You need to specify the

sweep

variable in the graph dialog box. Double click in the middle of the graph to bring it up:

You can do this also in ngspice:

plot i(vd1) vs i(vd2)

I usually place ammeter devices as @Rita suggested because the syntax for ammeter current is simple, while referencing a mos drain current is very complex and ngspice specific. If you port the design to Xyce it will not work.