Page lengths are taken from first US editions.
As Harry grows, so, too, does the length of the books (to a point). The films, on the other hand, are diverging more and more from the text of the books. The final book, Harry Potter and the Deathly Hallows, however, is being made into a pair of movies (expected release dates: 2010 and 2011).
Breaking the books into chapters, rather than pages, shows that author J.K. Rowling has been remarkably constant with her chapter lengths, varying between 18 and 22 pages per chapter. The first book, Harry Potter and the Sorcerer’s Stone, ran 309 pages, or 17 chapters. Chamber of Secrets is 341 pages/18 chapters. Prisoner of Azkaban: 435 pages/22 chapters. Goblet of Fire: 734 pages/37 chapters. Order of the Phoenix: 870 pages/38 chapters. Half-Blood Prince: 652 pages/30 chapters. Deathly Hallows: 759 pages/37 chapters.
Great graph, Ian. The later books tend to inaccurately dominate one’s memory of the series, so it’s good to reminded of how much shorter the early books were. (I’ve just used this to make a point to an author about not getting carried away and making a book too long!)
You need to fix a typo toward the end of your pages-vs.-chapters summary. You have “Half-Blood Prince: 652 pages/30 pages” and of course you mean “30 chapters.”