Here’s an old post, found by searching the COW under “wiggle” and “axis” — you can figure out how to apply it to a Y-axis wiggle. 🙂
To wiggle on the X axis only, enter this in the layer’s position property expression field:
foo=position.wiggle(4,100)[0];
[foo,position(1),position(2)]
You set up the wiggle expression, then substitute it for the first position value in the position array. The first(0) is X, the second(1) is Y and the third(2) is Z.
You don’t have to use “foo”. It could be any word or character that doesn’t have another purpose in the expression language. You could use “blat”, “xwigg” or “zac” if you like.
For more wiggling, go to the Adobe Studio Exchange.
Hope that helps,
Steve