Could be generated from things on a plot - Natural Wonders, Moai, and any number of things that I haven't thought of but others may.
It's as much a "yield" as culture and faith are
As I understand it, the way tourism works when plots are involved is from the buildings (Hotel, Airport etc.) and not the tile itself. So when you have the Moai giving two culture, it's the Hotel giving the tourism and not the tile. It's an absolutely stupid system, but that's the way they've implemented it.
I think a lot of the UI work is hardcoded - the Culture Overview doesn't support new GW buildings or unique versions of the existing ones that change the number of GW Slots, and IIRC the tech tree doesn't support y values for positions beyond what they've implimented (read: no scrollbar)