Radar charts are line charts plotted on a radial axis. They can be used with one or more datasets that contain at least three values each. There is only one axis, which starts from the center. Each line begins and ends at the same point and, for that reason, radar charts are usually used to display values that are either cyclic in nature (such as hours, months, schedules, or repeating events), a sequential list of categories which end at the same place where it begins (such as round-trip), or categories that have no specific order. A radar chart can be used to compare different datasets by revealing strong and weak points, or showing outliers and commonality in data. It usually works best with a small number of datasets (that is, no more than three or four).
Radar charts are usually a poor choice for large datasets. In these cases, it's usually better to use a Cartesian line chart or a bar chart. Radial distances are also harder to perceive, although this limitation can be minimized...