Graphing an N-channel MOSFET's characteristic curve in LTSpice: Am I doing it wrong?

Component in SPICE, even symmetrical ones like resistors, have pre-assigned current polarity when measuring current through them.

If you are measuring current through the voltage source it is negative via passive sign convention. If you want it to read positive you will need to add in a current sense resistor and position it the correct way and read the current through that.