This vivid example of data visualisation was created by John Snow and Charles Cheffins in 1854. Snow successfully traced the source of a cholera outbreak to a water pump. Each black bar on the map represents a cholera case.
Figure 6.4.1 John Snow’s map of the 1854 London cholera outbreak
Click on the map to zoom in/out:
Map from Snow, J. (1854). On the Mode of Communication of Cholera. London: C.F. Cheffins, Lith. This is a digitally enhanced version from the UCLA Department of Epidemiology website. Public Domain.