Lessons for the codefellows classes on d3js. Lessons are in branches.
The book:
http://chimera.labs.oreilly.com/books/1230000000345/index.html
Meaningful quotes from the book:
"That is the power of data()—being smart enough to loop through the full length of whatever dataset you throw at it, executing each method beneath it in the chain, while updating the context in which each method operates, so d always refers to the current datum at that point in the loop."