 This device is now known to have been used for sorting one target variable (e.g., crimes of different types) in relation to other possibly explanatory variables (e.g., population density).In 1829, Guerry invented a new graphic form, the polar area diagram to show variations
in weather and other phenomena over calendar cycles.This variation of the pie chart
uses equi-angular sectors of differing radii, in contrast to the pie chart that uses sectors
with varying angle, but equal radii.