Web Trend Map [IMG]

If you're on a mobile device, you might have a hard time opening the OP. Here's the smaller version: http://www.flickr.com/photos/formforce/3409362834/sizes/m/

Well, while the picture itself is pretty boring and I don't find it very useful or interesting (sorry), I really like the way it is drawn, and I would love to see a real transit map drawn like this, any hints how to do this easily? :)