How can I make an animation interpolate linearly between two keyframes?

I need to make a simple animation of a cylinder that rotates along the X axis. It should have 8 frames, and rotate a total of 216 degrees, i.e. 30.857 degrees per frame; the problem is that, although it does rotate the desired amount, it doesn't do so at a constant rate.

How can I make it always rotate the same amount every frame?