Case T-53/06: Judgment of the General Court of 6 March 2012 — UPM-Kymmene v Commission (Competition — Agreements, decisions and concerted practices — Plastic industrial bags sector — Decision finding an infringement of Article 81 EC — Duration of the infringement — Single and continuous infringement — Fines — Gravity of the infringement — Mitigating circumstances — Undertaking playing a passive role — Proportionality)