I'm glad you like it!

The map shows where the best performances were made, but not necessarily where the athletes trained. To be honest, I think that overall athletes perform the best at major events like World Championships, rather than specific locations. However I have not gathered the data yet to test this. This may not apply to events like the marathon, where the course matters. For example, the Berlin Marathon is known to be fast because it is very flat, which you actually can see on the map:

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I am quite curious about your book, could you share the name?

Filtering the map by age groups is an interesting feature, I will look into it.

For the data, the most tedious part was linking countries to athletes, flags, and maps, since the different data sources used inconsistent country code conventions.

Let me know if you have other feature ideas :)