Steppers and DC motors work on different principles. For a DC motor, the torque is produced by the force on a current-carrying wire in a magnetic field. The force is proportional to both the current and the magnetic field. So for a permanent-magnet motor, torque is proportional to current; whereas for a series-wound motor, torque is proportional to the square of the current (because the magnetic field is also proportional to the current, at least until magnetic saturation of the iron core sets in).
A stepper motor produces torque because of the forces between rotor and stator poles. Magnetic attractive force is proportional to the square of the field strength (if I said square root earlier, that was my mistake). However, on thinking about it, the square term probably comes in this case from the field strength of the permanent magnet rotor times the fields strength produced by the current in the stator poles. So I agree that a torque that is linear with respect to current seems very plausible. Nevertheless, I would still like to see a graph of actual measured torque vs. current.
A stepper motor produces torque because of the forces between rotor and stator poles. Magnetic attractive force is proportional to the square of the field strength (if I said square root earlier, that was my mistake). However, on thinking about it, the square term probably comes in this case from the field strength of the permanent magnet rotor times the fields strength produced by the current in the stator poles. So I agree that a torque that is linear with respect to current seems very plausible. Nevertheless, I would still like to see a graph of actual measured torque vs. current.