Rose Charts: Area or Length Encoding for Fill Level of Circle Sectors?

This paper examines the accuracy of value estimation from the fill level in circle sectors of rose charts, a circular chart type where all sectors share the same angle and encode values through either radius or area. But which encoding yields more accurate estimates? We conducted a user study comparing different sector configurations and chart sizes as well as the estimation errors of rose versus bar charts. Our findings indicate that both area and radius influence estimation accuracy. For values below 65\%, area dominates as a visual cue, whereas for larger values, a transition to length perception can be observed. Based on these insights, we propose a transfer function to correct for average estimation errors and provide practical guidelines for the effective use of rose charts.