This is a more realistic speed-torque curve. Notice that there are some speeds where the torque seems to drop to zero. This is due to the mechanical resonance points of the motor. A stepper is basically an under damped machine. Each time it takes a step, it over shoots and then settles in to the final position. Depending on the damping in the load, there may be some operating points that have a mechanical resonance that effectively drops the torque to zero at that speed.