Snow’s 1854 map of cholera deaths in proximity to the Broad Street Pump is one of several examples of attempts to visualize data about an epidemic’s spread. The other examples featured on this Princeton University Library site are fascinating too. See the online exhibit at large for other early thematic mapping projects as well as this UCLA page dedicated to Snow’s influence.
The site is mistaken about the date of Snow’s death: he died in 1858, not 1854.
Edward Tufte’s analysis of the map is incredibly telling as well (find the scan here: https://bit.ly/2VEqbgY ). Tufte juxtaposed it with the case of the NASA Shuttle Challenger disaster to show that data visualization in some cases is truly a matter of life and death.