: - MacGyver10
The source I'm using is Major Nelson's XBL activity numbers. Hence my claims that my data represents relative standing amongst games (and why I used the term "rank" in the title, and why the data goes 1-20). It's not a population graph, it's a graph showing where a Halo title was standing at a particular time relative to its contemporaries on XBL.
This information is very public.
But I could certainly add games from other series to the spreadsheet. Minecraft? ;)