How do I scale an object in only one direction? I am animating a volume meter, and have done the "tricky bit" of getting the volume value.

Now I want to apply the volume value to a mask , so that as the music moves, the mask is exposed in time to the beat.

I have tried two ways, the first by scaling the mask. However, I can't get it to scale in one direction only (i.e. it scales around a center point.) Even if I move the pan anchor it still doesn't pin the base of the shape.

The other way I tried was using a linear wipe. However, this was no good either, as the whole wipe effect is completed at 38% (i.e. if I move the slider the object vanishes totally at 38%, not 100% as I would expect.)

Is there a better way to do this?

Seems simple, but has me stumped.
Set the Anchor Point to the bottom of the object by pressing y or clicking on 
Disable proportional scaling 
Optional you can add a slider control to to get a better control of direction scale values




The shape can also used as track matte, just set the layer below to Alpha. 

Note: As alternative you could use a trim path on a shape layer as track matte, see this answer: 


https://graphicdesign.stackexchange.com/questions/49643/how-to-use-after-effects-trim-path-with-an-illustrator-path/49975#49975