Trim paths of rectangular shape layer wiping radially instead of vertically.

The Trim Paths animator is not designed to work on a shape layer fill. It only works in an easily predictable way on a stroke. There are other ways to create a wipe on or reveal transition on a filled shape. For a simple rectangle, you can try animating the offset as well as the start and end parameters of trim paths, but it's a little tricky to create a simple transition.

An easier workflow would be to just animate the size of the rectangle and the position or Rectangle/Transform Rectangle/scale and anchor point and or position in the Transform Rectangle properties. 

Another way is to use Merge paths with two shapes and have one of them animated. 

Another option is to just put one shape on a shape layer and just use Linear Wipe. 

I hope this helps. I should do a tutorial series on animating shape layers. There is no really good overview out there. They can be pretty complex very quickly.

To clarify, the title says vertically, but I want it to wipe horizonally, from left to right, or vice versa. Thanks!