Separating X and Y positional data is a cinch in AE. All that’s required of you is to right-click the position attribute of any layer, and select “Separate Dimensions.” This can be helpful for a plethora of things. For example, if you wanted to apply an expression to the position of a layer, but only wanted it to affect the Y axis, you can separate the dimensions of a layer, and write your expression on the Y axis attribute.
However, if you wanted to perform the same task, except on the SCALE parameter of a layer, you’ll find no such simple option given to you by After Effects. However, using a couple of slider controls and expressions, you can do it!
First, you need to add a slider control for each parameter you want to have individual control over. In this case, we are only dealing with the X and the Y scale, so we’ll need two slider controls. Name them whatever you want, or leave them as ‘Slider Control,’ it really doesn’t matter, but I recommend renaming them to keep organized. The slider controls can be applied to the layer you want to affect the scale of, or you can apply the slider controls to a Null object or Adjustment Layer. Hypothetically, the slider controls can be applied anywhere, but it would make the most sense on the layer they are tied to, or to a “Control” Null.
Next, option-click the stopwatch icon of the scale parameter to add an expression. Delete the automatically filled in text, and replace it with the following:
Now you have complete individual control over your x and y scale axes.
IMPORTANT! Whatever you rename your slider controls to must match within this expression. The two screenshots I’ve posted have matching names for my slider controls, so take note of their placement and change them as necessary.
You can also pickwhip the slider controls, but I’m too stupid to describe that in a text-based blog post. Maybe I’ll do some video tutorials, but I know you all just go to Andrew Kramer anyway 
I personally like to make colored solids and use this technique and apply an expression to my Y slider control and create dynamic looking sound bars. Of course, this requires re-positioning of the layer’s anchor point. See what you can come up with using this expression, I use it all the freakin’ time. I would say daily.
One more thing: I’ve been a lazy/busy douchebag, so I haven’t posted since February or something. But, I’m really going to try and keep it up this time, so stay tuned to hopefully more cool tips and tricks. THANKS
