Editing Pie chart (section)

===Polar area diagram===
[[Image:Nightingale-mortality.jpg|thumb|"''Diagram of the causes of mortality in the army in the East''" by Florence Nightingale.]]
[[File:2019 Carbon dioxide emissions by income group - Oxfam data.svg|thumb|Like conventional pie charts, a variable-radius pie chart has wedges whose areas represent  ''total'' quantities in respective categories/groups. However, here, each radius represents an amount of that quantity ''per unit'' within that category. In this example, each wedge's area represents total {{CO2}} emissions of all people in that category, and each radius represents emissions ''per person'' within that category.]]
The polar area diagram is similar to a usual pie chart, except sectors have equal angles and differ rather in how far each sector extends from the center of the circle.
The polar area diagram is used to plot cyclic phenomena (e.g., counts of deaths by month).
For example, if the counts of deaths in each month for a year are to be plotted then there will be 12 sectors (one per month) all with the same angle of 30 degrees each. The radius of each sector would be proportional to the square root of the death rate for the month, so the area of a sector represents the rate of deaths in a month.
If the death rate in each month is subdivided by cause of death, it is possible to make multiple comparisons on one diagram, as is seen in the polar area diagram famously developed by [[Florence Nightingale]].

The first known use of polar area diagrams was by [[André-Michel Guerry]], which he called {{lang|fr|courbes circulaires}} (circular curves), in an 1829 paper showing seasonal and daily variation in wind direction over the year and births and deaths by hour of the day.<ref name="Friendly, p. 509">Friendly, p. 509</ref> [[Léon Lalanne]] later used a polar diagram to show the frequency of wind directions around compass points in 1843. The [[wind rose]] is still used by [[meteorologist]]s. Nightingale published her rose diagram in 1858. Although the name "coxcomb" has come to be associated with this type of diagram, Nightingale originally used the term to refer to the publication in which this diagram first appeared—an attention-getting book of charts and tables—rather than to this specific type of diagram.<ref>{{cite web | url=http://www.florence-nightingale-avenging-angel.co.uk/GraphicsPaper/Graphics.htm | title=Florence Nightingale's Statistical Diagrams | access-date=2010-11-22}}</ref>