I typed it out manually in Vim using its built-in digraphs to get the box drawing characters: <C-K>vv for the vertical line, <C-K>vr for the vertical-and-right line, <C-K>ur for the up-and-right line.

See also https://news.ycombinator.com/item?id=30273299 for related discussion yesterday and one popular tool that helps with related things, if not this specific style of illustration.