fuck it, let's go
Unfortunately, we need to consider each axis separately. So for each axis, we're comparing the relationship between four numbers: the min and max of a, and the min and max of b. 
Some of the moves are obvious: when layers are aligned on an axis and the other axis is disjoint, then they can have a straight line. But things get trickier with overlaps and contains.
Each rectangle has four connection points at the midpoint of each edge, plus eight points padded out around it. My hunch is that if I got the angle between the two rectangles, then I could compare these points to get the right connection shape.
I'm also doubling my work here by including right-to-left and bottom-to-top arrows. I can skip these by just running everything in two directions, once I've decided which rectangle is topmost/leftmost.
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