wiggler expression for position

I want to have a random movement on a layer using the a wiggle expression but I only want it on one axis.

Using this:

wiggle(1,50)

makes it pop around on the screen, so what do I need to add to this to get it to work up and down only?

Also, is there a 'library' of sorts that anyone can point me at that has some useful expression examples to get me started.

I realise thats a little like 'how long is a piece of string' but its worth asking!

Thanks