John Snow and a section of his famous map depicting the pumps in London contaminated by Asiatic cholera, 1848. These cases were linked to a single provider of water for that pump, and spurred on numerous future attempts to use mapping to define causes for various diseases.
John Snow's data journalism: the cholera map that changed the world
In 1854 John Snow mapped the cholera outbreaks around Soho. His map made apparent how the cholera cases were linked to the local water supply. This is probably one of the first "data" maps created to systematically find a solution to a problem.
The Ghost Map: The Story of London's Most Terrifying Epidemic--and How It Changed Science, Cities, and the Modern World, by Steven Johnson is a very interesting book. The pump handle! This book has it all!