This is an interactive map displaying the nationalities of the people working on Marvel's comics and how this evolved over time.
It shows the number of people from a certain country who worked on a Marvel comic in a given year. Which means that a writer who's written 40 comics in a year has as much weight as an artist who was only hired to do a cover. Such is life.
The data I've used comes from ComicVine's database. I've done my best to correct and complete it, but there are credited people for whom I don't know the role or their nationality so it's not an exhaustive resource.
I've counted as artists not only pencilers, but inkers, colorists and letterers as well.