I have seen a couple of papers where they plot intermodulation distortion % versus collector current in a BJT.  

Does this mean that they are doing a nonlinear simulation for a constant current source applied to the collector?  Or does it mean that they have a voltage source at the collector and are plotting versus the DC component of the collector current?

A paper which describes this is ``Distortion in Elementary Transistor Circuits'' by Willy Sansen.  It is in IEEE Trans Circ and Syst, Mar. 1999, p 315--325