What a mess this was! We have two nearly-identical BackupPC setups and one wasn't graphing but the other was. Research on the interweeb, and chats on IRC, didn't seem to give any results so hopefully this can help someone. Note that the /pool/ partition on the failing-to-graph system is ReiserFS 3, which may have been a contributing factor. Also the Pool size wasn't ever decreasing meaning something wasn't being cleaned up.
Got the info from this list
http://www.backupcentral.com/phpBB2/two-way-mirrors-of-external-mailing-lists-3/backuppc-21/backuppc-doesnt-clean-cpool-94508/
"
In 3.1 an optimization was added to use IO::Dirent for reading
the inodes in a directory, which on certain filesystems doesn't
work correctly. If you are running 3.1.0 I would recommend
trying to disable IO::Dirent by changing this line:
$IODirentOk = 1;
at line 83 of lib/BackupPC/Lib.pm to:
$IODirentOk = 0;
"
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