from the formula of BlackMercedes, you can see that the HP varies linearly with torque or RPM, depending on which you are holding constant.

in the ideal case, where you have a perfectly flat torque "curve", i.e., the torque is constant from 0 rpm to redline, the engine power will be linear with RPM - it will also be a straight line starting at 0 from 0 rpm to the max power at redline.

in the real world, an engine designer could only hope for a "torque plateau", spread over as wide an RPM band as possible, over an RPM range where the engine will be spending most of its time.

With this "flat torque curve", the torque stays relatively constant over a wide range of RPM. This is the engine's "sweet spot", where HP varies linearly with RPM. From a driver's perspective, you get a very proportional and predictive response from the engine, as you increase RPM.

However, the fly in the ointment is aero drag, which increases with the square of the speed, so even though an engine may have a linear power curve all the way to redline, the performance of the car will be less than linear.