According to [0], the illustrations in "ØMQ - The Guide" [1] are made using asciitosvg. See, for example, the illustration following the "Divide and Conquer" heading:
http://zguide.zeromq.org/page:all#Divide-and-Conquer
[0] https://9vx.org/post/simplistic-diagrams-with-ascii-art/