Took me a bit to get back to this, been a busy day. So I messed around with this and it’s basically a math problem. You have the null rotating at 9 degrees every frame with a large velocity. Even from your picture, you can see that over 20 frames you get 20 lines, each 9 degrees apart. To get a smooth line you would have to adjust your frame rate 9 times faster to get a 1 degree move, or 180 frames per second but AE only goes up to 99 frames per second.
To get the animation you want, you need to move your keyframes to 6 seconds(9 times the length it currently is). Precomp the entire thing and time remap.
Johnny Cuevas, Editor
http://www.thinkck.com