Is it possible show difference between two graphs in cacti

What does

% munin-node-configure --suggest | grep apache
apache_accesses            | no   | no [Port 80: Forbidden]                
apache_processes           | no   | no [Port 80: Forbidden]                
apache_volume              | no   | no [Port 80: Forbidden]    

return ?

Aside from upgrading to the latest 0.8.8, have you confirmed that you (or whatever is attempting to store the rrd information in the .rrd file) can? Are permissions for the file/directory set properly to allow for this?

In reply to your comment that file/directory ownership/permissions may be the issue, I will share what I have found to be useful for my Cacti installations that run on OpenBSD (aka, your placement and syntax may vary with other systems):

chown -R www:www /var/www/htdocs/*
echo 'web directories reset to www:www ownership'
chmod -R 777 /var/www/htdocs/cacti/plugins/*
chmod -R 777 /var/www/htdocs/cacti/log/*
chmod -R 777 /var/www/htdocs/cacti/rra/*
echo 'cacti plugin, log, and rra directories set to full r,w,x'

I use weathermaps heavily in cacti and often find myself having to reset permissions over and over throughout and this has save me the headache of thinking about it constantly.

777 may be too open for these sections security-wise, and if so, I would gladly appreciate a better setting to be mentioned that still provides usability of the application.