We had intended to try the Victor with a purely resistive load but didn't have time before we had to go to "build" mode.
Seeing that plot leads me to believe that the source of the nonlinearity is the back EMF from the motor. If your turn the shaft on a DC motor, it turns into a generator. When you drive it as a motor, it also acts like a generator, however, the sense of the voltage is inverse to the supplied voltage. This is why stall current is the maximum current the motor sees and then drops with speed. It could also be that speed is proportional to power and power it proportional to Voltage squared or Current squared.
I've also noticed a lag in seeing the motor start after an output is applied to the Victor and hence the motor. I've yet to do the experiments to determine if it's in the Victor repsonding to the input or if it's the rise time of the magnetic field in the motor.