From Diagrams that changed the world (BBC News):
One of the first to use the visual world to navigate numbers was Florence Nightingale.
Although better known for her contributions to nursing, her greatest achievements were mathematical. She was the first to use the idea of a pie chart to represent data.
Nightingale had discovered that the majority of deaths in the Crimea were due to poor sanitation rather than casualties in battle. She wanted to persuade government of the need for better hygiene in hospitals.
She realised though that just looking at the numbers was unlikely to impress ministers. But once those numbers were translated into a picture – her Diagram of the Causes of Mortality in the Army in the East – the message could not be ignored. A good diagram, Nightingale discovered, is certainly worth 1,000 numbers.