Excellent charts function to give viewers a clear understanding of the underlying data. It allows anyone to grasp meaning and identify trends and patterns. This is particularly important in times of war.
Florence Nightingale used the Polar Area Chart to show the extent of needless and preventable deaths in British military hospitals during the Crimean War. The Diagram of the Causes of Mortality was published in 1858.
The legend reads: The Areas of the blue, red, & black wedges are each measured from the centre as the common vertex. The blue wedges measured from the centre of the circle represent area for area the deaths from Preventable or Mitigable Zymotic diseases, the red wedges measured from the centre the deaths from wounds, & the black wedges measured from the centre the deaths from all other causes. The black line across the red triangle in Nov. 1854 marks the boundary of the deaths from all other causes during the month. In October 1854, & April 1855, the black area coincides with the red, in January & February 1856, the blue coincides with the black. The entire areas may be compared by following the blue, the red, & the black lines enclosing them.
The polar area diagram is similar to a pie chart, however all sectors have equal angles and differ in how far each extends from the centre. This type of chart works very well for cyclical data. It was invented André-Michel Guerry, a French lawyer and statistician, in 1829 to show variations in weather over calendar cycles.
Nightingale used charts like these extremely effectively to lobby for change in public health.