Add MASQUERADE Rule on Proxy server for your PC IP Address.
Suppose,if you have a LAN of 100 PC's & except your PC,all 99 computers getting internet through Proxy Authentication.
Now,to access internet from your PC through Proxy without authentication,then you need a single "iptables" rule on your PC's IP Address.
Just implement below rule on proxy server & you will get internet directly,without any auth.
The command is :-
iptables -t nat -I POSTROUTING -s <your pc ip address> -j MASQUERADE
I have tested that this rule can work on RHEL,Fedora,CentOS only.
Try it,it should work.