To dovetail off another HN thread (Free Springer math books)...after reading Bostock's piece, I suddenly realized that the classic Grammar of Graphics by Leland Wilkinson he quotes might be free...and it is! The 1999 version anyway:
http://link.springer.com/search?query=grammar+of+graphics
On Amazon, the cheapest available price is ~$55. For the second edition, which was printed in 2005 (and should be due to be free soon on Springer), it costs $132+