Calculating amperage in a current source
The current through the '?' that you have highlighted is the same as the current through the base-emitter of the transistor.  The base-emitter of the transistor is a diode and that is the characteristic that you are seeing.  The base-emitter diode is effectively shorting out your 1k resistor, so determining the current through the base-emitter is difficult.  Do some reading about the diode voltage-vs-current to get a better idea of the causes of the curve you are observing.

A simple current source can be found on wikipedia if that is the type of circuit that you are looking for:



If you are just looking to experiment to see where things go, I would suggest placing a static resistor where you have the pot and moving the pot as a variable resistor between the voltage divider and the gate so that you can more directly alter the current through the transistor.