debian 4.0_rc1 install, the stats graphs hadn't updated since mid july. looking at /opt/zimbra, it turns out there are two areas for rrd and associated graphs, /opt/zimbra/logger/db/work and /opt/zimbra/zimbramon/rrdtool/work. the zimbramon area hadn't been updated since aug 11 and running zmgengraphs didn't update it, although the zimbra_core package referred to two gif files in there so can't have been completely redundant. running zmgengraphs does however update /opt/zimbra/logger/db/work rrd and gif files although these were clearly not the graphs being displayed through the admin interface. moving the zimbramon dir out of the way and symlinking the logger directory however seems to have done the trick and now i get up to date graphs in the admin interface.
looking at another rhel4 installation i updated a few days ago it looks like a similar thing has happened - the zimbramon/rrdtool/work hasn't been updated since the 4.0_rc1 update was applied a few days ago, and it looks like the graphs also haven't updated since this update, and the admin interface is showing the graphs from the old directory.
bug 10093 filed