Consider replacing rectangle x/y with index/value

[benmccann]

There are a lot of checks for horizontal in element.rectangle and controller.bar and I think we could make almost all of them go away if we used index and value instead of x and y

[etimberg]

@benmccann do we still want to consider this in v3?

[benmccann]

I still think it's a good idea. But I probably won't have time to work on it personally

[etimberg]

Closing since we aren't going to change this for v3