Sophos sarg

Step 1: On the appliance:

configuration / system / backup

enter in the ftp information and a user account.

select "transaction log files at least once per day"

select squid format

this will upload the sophos_log to your ftp server.  Once you have the file see this document and split out what ever elements you wish to report on:

Step 2: http://swa.sophos.com/webhelp/swa/concepts/InterpretingLogFiles.html

then you can create any type of report you like, or just dump the separated fields out into a database.

ie: https://www.tecmint.com/sarg-squid-analysis-report-generator-and-internet-bandwidth-monitoring-tool/

most of the properties are already listed but you could add all of the ones on the above link as well..