Broad Street Cholera Map

John Snow’s 1854 dot map of cholera deaths in London’s Soho district is a founding document of both epidemiology and data visualization. Each black bar represents a death, stacked at the address where the victim lived. The spatial clustering of deaths around the Broad Street water pump made the waterborne transmission hypothesis visually undeniable.

The map didn’t just display data — it changed public health policy. After Snow presented his evidence, the pump handle was removed and the outbreak subsided. It demonstrated that visualization could be an instrument of persuasion and action, not merely illustration.

Snow’s map is a masterclass in the dot map technique: simple marks, precise geographic placement, and a pattern that emerges from accumulation rather than from any imposed structure.