A fan chart is a line chart that reveals ranges of possible values, and multilevel half-pie chart that reveals hierarchical relationships. The line chart style spreads in a fan-like shape as time progresses to reflect the uncertainty of the future. The multilevel chart style assigns a quantity of groups in the first inner ring, and as the chart moves outward, each subsequent ring segment is divided into sub-groups.
Their usage (in line style) was popularized by the Bank of England through their “Inflation Report.” In this report which targets the general public, the bank describes what they believe future inflation will be. The bank along with the finance industry and bodies that govern or manage monetary policy have used the charts heavily for many years, particularly for forecasting.
The line style predicts future values in the form of upper and lower ranges, or through intervals. The shape of the forecasting distribution will dictate the way the forecast density is represented. If the density is symmetric, the center of the fan will be the mean and the ranges expand in a way similar to confidence intervals; equal-tail ranges. If the density is not symmetric, the center of the fan will be the mode and the ranges will use the highest probability density.
The hierarchical fan chart reveals relationships. It is employed across disciplines and can be used to describe things like relationships in languages, species, or organization ranks. They are commonly used to express genealogy and to express relationships in software or computer storage. They have a clear advantage over other options like an organization chart, which is that they can present a large set of information clearly in limited space. Their capabilities and versatility can also be expanded through software such as software which allows annotations and reveals deeper relationship data that cannot be observed.