[klyrs]

Such an innocent-sounding problem, but I've drawn tons of figures in inkscape, tikz and matplotlib and I swear, I'd never stop fidgeting with curved arrows if time was immaterial.

But of course, I have Opinions that would keep me from using this in practice. For example, my favorite style is similar to a half curly-bracket (or, taste depending, an integral symbol) -- two small semicircles connected by an axis-aligned line.

Edit: oh, and question from the peanut gallery... what if one box is fully within the other, and centered?

[javajosh]

>I have Opinions

Good. Then fork the library and modify it to your liking, optionally republish.

>one box is fully within the other, and centered

This implies containment/stacking and wouldn't require an arrow. However, you could do worse than drawing 4 arrows from the edges of the surrounding rectangle.