The proxy authentication is pretty much useless unless the proxy log also shows the usernames;
users can login from mutliple devices (laptop, smartphone, tablet) with different source ips, so the source ip alone doesnt tell much at all; furthermore after the dhcp leases have expired its impossible to associate the source ips to mac addresses.
The attached patch works for me and shows the usernames (which are already logged in the squid access logs) with logproxy.cgi; it is also possible to filter for usernames in addition to source ips.
The patch also fixes the issue where the 1st call of logproxy.cgi shows up with 'Enable ignore filter' unchecked while the button 'Restore defaults' sets 'Enable ignore filter' checked.
Please consider adding this feature for future releases.