In the olden-days, when “epidemics” were frequent and really meant something, cholera was wrecking havoc on London. Unsure where the disease was originating, John Snow mapped the residence of folks who contracted cholera. Soon enough, it was clear that the Broad Street well was the source of the outbreak—despite inconclusive direct evidence of chorea in water samples.

There were some outliers, however. After some detective work, a pair of outliers were explained:

[she] had lived in the Broad Street area at one time and liked the taste of the water from the pump so much that she had bottles of it brought to her regularly. Water drawn from the pump on 31 August, the day of the outbreak, was delivered to her. As was her custom, she and her visiting niece took a glass of the pump water for refreshment…

Today, many designers would likely use a bubble plot to display density on a map. But frequent overlapping can be confusing, this approach is much better to show correlation around a particular point.