...the normal Blueprint style is just three boxes: a long box and then two short ones. However, Plumb will create two boxes for the rows and then put the two short boxes in the second row. Plumb should create fewer elements.
Plumb needs better testcases. The master branch has some tests for stretchy layouts which kind of work, and the blueprint branch has a test for overlapping which fails (and at this point I don't really know why). One problem with testcases for a visual tool is that if you write down some coordinates, it's pretty hard to know what they mean a while later. "ASCII art" is one option?