In 1989 the IAAF introduced random drug testing.pic.twitter.com/53ybBQa2n8
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Hard to tell without looking at the graph's source data, but that would still be wrong. For instance, marathons existed at the beginning of the period covered, and the previous example I mentioned was an example of a marathon world record in 2005, but that's a zero on the graph.
I would guess what the IAAF counts as an event roughly tracks what the Olympics count, and the Olympics introduced the women's marathon in 1984. If you want more detail, why not ask the author of the article?
