You have something like ``"[WARNING] Could not draw graph "/var/lib/munin/cgi-tmp/munin-cgi-graph/"`` in your `/var/log/munin/munin-cgi-graph.log` ???

**Check the directory permissions!**

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

shows something like

	drwxr-xr-x 3 munin www-data 4,0K Mär 29  2013 munin-cgi-graph/

means the web server (group) has no write to this directory but graphs are rendered by cgi by the web server. So give the web server sufficient rights.
Do

	chmod g+w /var/lib/munin/cgi-tmp/munin-cgi-graph/
	ls -l /var/lib/munin/cgi-tmp/

now it should look like

	drwxrwxr-x 3 munin www-data 4,0K Mär 29  2013 munin-cgi-graph/

et voilá!
Graphs are drawn like a charm.