I made an error. The stats-graph.php script doesn't need a fix.
Obviously an other thing I did made my statistics work. I opened a shell on the server and manually ran the pack-sysstats script.
I tried this:
# cd /usr/local/psa/libexec/modules/watchdog/cp/
# ls -al
total 44
drwxr-xr-- 2 root psaadm 4096 Jan 5 16:57 .
drwxr-xr-- 6 root psaadm 4096 Mar 13 2009 ..
-rwxr-xr-- 1 root psaadm 408 Sep 12 2008 clean-events
-rwxr-xr-- 1 root psaadm 601 Sep 12 2008 clean-reports
-rwxr-xr-- 1 root psaadm 442 Sep 12 2008 clean-sysstats
-rwxr-xr-- 1 root psaadm 2769 Jan 5 15:43 pack-sysstats
-rwxr-xr-- 1 root psaadm 2697 Sep 12 2008 pack-sysstats.old
-rwxr-xr-- 1 root psaadm 543 Sep 12 2008 secur-check
-rwxr-xr-- 1 root psaadm 10951 Sep 12 2008 send-report
# ./pack-sysstats
Usage: pack-sysstats day | week | month
# ./pack-sysstats day
# ./pack-sysstats week
# ./pack-sysstats month
# ./pack-sysstats year
# cd /usr/local/psa/admin/htdocs/modules/watchdog/
# ls -al
total 104
drwxr-xr-x 3 root root 4096 Jan 6 08:26 .
drwxr-xr-x 4 root root 4096 Mar 13 2009 ..
-rw-r--r-- 1 root root 782 Sep 12 2008 disk-edit.php
-rw-r--r-- 1 root root 1130 Sep 12 2008 disks.php
-rw-r--r-- 1 root root 274 Sep 12 2008 index.php
drwxr-xr-x 3 root root 4096 Mar 13 2009 locales
-rw-r--r-- 1 root root 7400 Sep 12 2008 preferences.php
-rw-r--r-- 1 root root 9799 Sep 12 2008 secur-scan.php
-rw-r--r-- 1 root root 817 Sep 12 2008 service-edit.php
-rw-r--r-- 1 root root 1061 Sep 12 2008 services.php
-rw-r--r-- 1 root root 23785 Jan 5 15:43 stats-graph.php
-rw-r--r-- 1 root root 23576 Sep 12 2008 stats-graph.php.old
-rw-r--r-- 1 root root 1850 Sep 12 2008 stats.php
But like BrysonC the image on Statistics page is showing nothing anymore. This is a Plesk 8.6 server on CentOS 5.5 with MySQL 5.1.52 and PHP 5.2.14 within a Virtuozzo 4 environment. Anyone?