A Coxcomb chart, sometimes referred to as a polar area chart or a rose chart, is sort of a combination of a bar chart and a pie chart. Instead of the angles being changed based on data, the area of each segment is adjusted based on data by changing the radius. It is important not to change the radius purely based on the data, but to proportionately change the radius so that the area of each segment changes with the data. Changing the raw radius value will disproportionately change the area, causing people to perceive the data incorrectly.
The invention of the Coxcomb chart is attributed to Florence Nightingale, a nurse during the Crimean War. (Interestingly, Nightingale doesn’t seem to have ever referred to them as Coxcomb charts. Instead, she called the books that contained the charts Coxcombs, and the name stuck to the charts themselves.
Florence Nightingale was one of the first to establish the importance of sanitation in hospitals, and she collected meticulous data about the relationship of cleanliness to death tolls. When she presented her data in 1858, she developed a Coxcomb chart, a variation of a pie chart that represented the number of soldiers using the area of the circle segments instead of the radius. Her message -- that more soldiers died from infection than from wounds -- could be easily seen and resulted in policy changes that helped save lives.
In a Coxcomb chart, each category is represented by a section of the disc, and each section has the same angle. The area of a section represents the value of the corresponding category.
One problem with Coxcomb charts is that subtle differences are hard to see, so they are best used when trying to demonstrate patterns rather than delivering exact numbers.
But because Coxcomb charts make for a nice image without any decorative elements, Coxcomb charts can be a good choice for presenting information to viewers who don’t have much technical knowledge about the information being presented.