Define coordinate system

[mwadams]

As far as I can tell, the spec doesn't define the coordinate system in use (e.g. which direction is positive in each axis, what point on the entity is represented by its {x,y} position.

For interoperability, it would be good to tie this down explicitly.

[chainlist]

I think it's safe to assume this coordinates system is based on the webview one.

x axis: negative values go left, positive right
y axis: negative values go up, positive go down.

But I agree, it should be said explicitly

[mwadams]

And to clarify which point does it specify on the node: Top-left? Center? Bottom-left?