Munin could not draw graph

Make sure that /var/lib/munin/cgi-tmp/munin-cgi-graph/ is writable by web user/group. You can check with following command:

ls -l /var/lib/munin/cgi-tmp/munin-cgi-graph/

As long as you are using CentOS and Apache, you should be able to fix it with following commands:

chown -R munin:apache /var/lib/munin/cgi-tmp/munin-cgi-graph/
chmod g+w /var/lib/munin/cgi-tmp/munin-cgi-graph/

More info here: https://gist.github.com/viegelinsch/49c404b1b99a462bad4f

Additionally, you may need to change init script for munin-fcgi-graph because default configuration in package do not set permissions properly - reference:

Note, munin-fastcgi-graph should really run as user www-data (or your distrib standard httpd user), otherwise you'll open write access to the CGI process. The user www-data should be able to read the state files, the rrd and write to the cgi-tmp dir.