Ok, so from your reply we can assume the plugin works when run as root.
afaik see all files have the correct permissions to run this thing
sudo -u nagios /usr/lib64/nagios/plugins/check_mrtgtraf -F /var/www/html/mymrtg/10.0.1.1_2.log -a AVG -w 1000000,1000000 -c 5000000,5000000 -e 10
If that gives correct output, the error you told us about in your first post is either from an old log line or another object, because if the above is all true it SHOULD work.
If not, you forgot something to tell us