Lines shown are unique viewport heights. That means that no matter how many visitors have the same viewport height, the line will only be drawn once.
This is meant to show the diversity of viewports, not the popularity of them.
On page load, I grab the viewport and send it back to the server. Due to abuse however, I limit the number of requests a single IP can make. I know this limits the number of "folds" that will be drawn (e.g. any agency/company/whatever will only have a few IP addresses, so multiple visitors from within the same network will only register one line), but I couldn't think of a simpler way to limit abuse of the site.
Think this is cool/awesome/meh? Let me know on twitter . Full write up coming soon on my personal site, iest.co .
Update 9th Feb 23:39GMT : I've changed the site slightly so the lines are now drawn from a randomised subset of the full data, and the lines are now rendered by client-side javascript to stop my server from melting.