As a matter of fact if
you have Squid installed with snmp and
you can carry out an snmpwalk successfully you only need to do
snmpbulkwalk -v2c -Os -Cc -c communitystring hostname:3401 cacheClientHttpKb
snmpbulkwalk -v2c -Os -Cc -c communitystring hostname:3401 cacheClientHTTPHitKb
To tell you exactly who's generating the most traffic (through the squid cache).
The first command lists all the cache clients and their total traffic from squid and the second lists all the cache clients and the cached traffic from squid.
NB all the OIDs (like cacheClientHTTPHitKb) available are listed here http://wiki.squid-cache.org/Features/Snmp
Instead of mucking about for hours and hours getting a massive tool like cacti to (fail to) generate the graphs I wanted I redirected the outputs of these commands to a file in a cron job and parsed the files in php and then used the php extension "libchart" to graph them. Took me about an hour, and I still may post that how to.