This graph shows the total number of open files. This can be helpfule to determine if there is a resource-hog process and if the operating system limit is hit. The data for it is returned by the nagios plugin check_open_files.pl. The plugin Copyright (c) 2007 Christophe Thiesset, is available here: http://exchange.nagios.org/components/com_mtree/attachment.php?link_id=3328&cf_id=24