Timeline: add x-axis label for dates

[ehrhart]

Arno suggested to add a label explaining what are the dates (years) displayed on the timeline.
Is it the date of publication? or creation? Does it depend if it's a text or an image?

[rtroncy]

For images, this is surely the creation date (of the painting).
For texts, this is a good question, but I suspect it is the publication date (I do not know how frequently the books we have have been regularly republished in various editions though). @pasqLisena Do you know?