Sounds like burn-in at that bar. I don't think gravity could have any such effect. Plasmas are subject to burn-in, which is permanent and can be considered the death of the TV, IMO. Saw the exact same issue at the QVC store at the Mall of America, where they show their own channel all day long. Greenish burn-in ruined their first set of TVs. You could see part of their sales bars 100% of the time in green haze. Not exactly sure why it was greenish, I'm sure there's a reasonable explanation. Last time I was dragged there I noticed they had replaced them with LCDs instead.