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.