Looking through the latest Society of Illustrators Annual, I found the perfect illustration of a visibility graph on the cover of a New Yorker from two years ago, by Richard McGuire.

Richard McGuire cover illustration for The New Yorker, February 7, 2005, showing lines of sight between people in a crowd

(I have no connection to this site, but since I stole their image instead of taking the trouble to scan it myself, I'll give them a link: prints available for sale here.)