In this book chapter we discuss the differences and commonalities of radial and Cartesian visualizations. Though radial diagrams are oftentimes not seen as efficient representations of data they sometimes do a good job. They are not as space-efficient as Cartesian visualizations but in many cases they benefit from aesthetics. In some scenarios they are also very powerful when there are no explicit end points on a data axis. A circular shape can be used to close this gap by putting formerly existing end points in a Cartesian representation together. Also for visually encoding circular or periodic time radial diagrams are a good design, sometimes also extended to spiral representations meaning several circles radially stacked on eachother with some increasing radius depending on the period.
Typically, pie charts are regarded as a bad design as explained by some researchers in the field. A pie chart mumbles when it speaks, as coined by Stephen Few. The drawback is the poor comparability of quantitative data values due to the non-aligned scales.