#24 per-book graph pluglets

[T.J. Willis]

Here's one for the statistically-oriented: there are
several feedback pluglets that graph statistics on a per-
chapter basis. It occured to me that it would be nice to
see the same types of information on the current book.
For example: an area chart with the aggregate number
of incorrect answers in each of the book's chapters in
red, overlaid with the number of correct answers in
blue. The code would be very similar to
HistoryGraphPluglet
(http://cvs.sourceforge.net/viewcvs.py/pavlov/pluglets/p
lugsrc/history/HistoryGraphPluglet.java?
rev=1.2&view=markup)