Finally finished. Question: I see I can inspect the canvas element and even edit the code. How do I run it with my edits (without pushing it to a server?)
1) Go to the Gist that this bl.ock refers to: https://gist.github.com/mbostock/11357811
2) Fork that Gist and make your changes.
3) Go to bl.ocks.org/niels_olson/11357811 to see the finished product.
bl.ocks just displays whatever Gists you have up on GitHub, so you don't have to make an account or anything.