I found that xgress tracker has free access for 2 weeks. Ive tried to install plugin from their site and it looks very good. Is it legal to use additional plugins for IITC like this one?
No, absolutely not. The linked site is scrapping Ingress which is 100% against the TOS. Using plug ins from that site are also 100% against the TOS and can get you banned easily.
dont ever use that website, they are scraping data and its illegal , breaks TOS....
Can you comment on why this is illegal?
In any case, those who use it will never be banned because it cannot be tracked. Here we are talking about general principles of use that many of us do not share.