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How does it look? Are they always axis-aligned or always tilted?

In most cases pages are too crowded as they are, not sure if I'll be able to squeeze a legend. And in some cases  it may be quite long: https://dysonlogos.blog/2013/12/23/the-key-to-all-this-madness/!

(+1)

Sorry for the super late reply. I think maybe it could be something like a number, like you did for the city 2D maps and then the legend of those numbers could be writen elsewere on the page, outside the map itself. (Sorry for my bad english, I don't know if it makes any sense). As for the look question, to be honest I do not really have an answer, but what I "see" in my mind it's a number just at the bottom right for exemple (but i could be at another positon) of the object that is anoted, or maybe right in the middle of the object.