In that case, the map looks misleading, doesn’t it? It looks as though London and Berlin, for example, stand out as having dislocated disproportionately many artifacts from around the world. And on the other hand, Paris looks innocent. But in truth, London and Berlin are just being honest about it giving you open access to the data whereas Paris doesn’t. So perhaps it would be better to mark cities that do provide data with, say, a green dot. This also helps the visual understanding of the direction of the transfer. And it would make it clear at a glance that Paris is not innocent, but that Paris is simply lacking data.

Fair point, and a good idea. A visual marker distinguishing "has open data" from "doesn't" would meaningfully reduce that distortion. Adding it to the list.

> London and Berlin are just being honest about it giving you open access to the data

A small subset of the data. If all London museums were included (such as the British Museum, for example) there would be a LOT more lines going to London.

I was slightly discombobulated by the first London link I clicked actually showing a number of artifacts from London that are currently held in Berlin.