In the nineteenth century, it was believed that cholera was transmitted and spread by miasma (a theory that claimed epidemics were caused by bad odours emanating from rotting organic matter). In 1854 a major outbreak of cholera reached the district of Soho, London. A lack of proper sanitary services and poor drainage meant that the outbreak hit hard.
John Snow was an English physician and a sceptic of the miasma theory. By visually representing the location of each cholera case on a map, Snow was able to show evidence of a connection between the Broad Street water pump and the number of cholera cases in the immediate vicinity.
This map presented the data visually and geographically, allowing us to see a pattern and correlation between the water pump and location of cases. Not only did this insight ultimately lead to the discovery of the source of the outbreak, it forever changed how we interpret our world.