Big Cartesian triangle example

[zmoon]

As somewhat of a uxarray newcomer, based on these vertices and units of meters, I would respectfully disagree that this is a big triangle (taken from here).

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I think it may need to be made more clear that it's the normalized Cartesian coords that matter in this situation. Also latlon is not documented in uxarray.open_grid.

Also, I wondered if it might be worth adding at least an option to the Matplotlib collection methods, to support making that plot automatically. That is, at least in the case that the ax is set to plate carrée projection, allow the edges to be presented in a (pseudo-)plate carrée way instead of geodetic.

I could take on this issue.