Good guesses, but wrong so far. Here are the rest of the hints, separated by whitespace. The complete spoiler, plus some related graphs, can be found here: http://goo.gl/2veWy Hints below: Hint #1: The X axis goes from 2..10,000, and the Y axis from 0..1. Hint #2: The graph is a subset of an OEIS sequence credited to Erdos and others, with the Y axis scaled to show proportion rather than absolute quantity. Hint #3: The set of X values where the vertical lines reach the top are also a well known integer sequence. Hint #4: An image related to this graph can be found here: http://lunkwill.org/mystery.png