Precise Keyframing in Final Cut Pro X
You cannot expand the Transform Line in the Timeline. You can finetune your Position keyframes separately in the Viewer.

Create keyframes and navigate between them with the keyframe buttons in the top-left corner of the Viewer.



Position keyframes

Click the Transform button in the lower-left corner of the Viewer (Shift-T).


The yellow handles in the middle of your clip are your position keyframes. Control-click on one of them.


Choose Smooth in the popup, you will see two smaller red dots, they are the bezier handles!


You can change direction and speed by moving the bezier handles.

change the length of the handle that points to the other keyframe
short means fast, long means slow
set your start speed in the first keyframe, and your end speed in the last keyframe.
if you make them both short, you will start gradually, speed up in the middle and slow down before stopping. use it for a smooth motion
if you make them both long, you will start promptly, slow down in the middle and stop abruptly. Use this effect to imitate gravity (bouncing)



Rotation and Scale keyframes

These can also be set separately but they are not visible on the Position line in the viewer as yellow dots.
In the middle of your clip is the blue Rotation handle.
On the corners and borders are the eight blue handles for Scaling.
Scale and Rotation keyframes don't have a smooth option in FCPX. It might be possible to do that in Motion.