  Radial charts are generally considered less effective than linear charts.
Perhaps the only exception is in visualizing periodical time-dependent data,
which is believed to be naturally supported by the radial layout. It has been
demonstrated that the drawbacks of radial charts outweigh the benefits of this
natural mapping. Visualization of daily patterns, as a special case, has not
been systematically evaluated using radial charts. In contrast to yearly or
weekly recurrent trends, the analysis of daily patterns on a radial chart may
benefit from our trained skill on reading radial clocks that are ubiquitous in
our culture. In a crowd-sourced experiment with 92 non-expert users, we
evaluated the accuracy, efficiency, and subjective ratings of radial and linear
charts for visualizing daily traffic accident patterns. We systematically
compared juxtaposed 12-hours variants and single 24-hours variants for both
layouts in four low-level tasks and one high-level interpretation task. Our
results show that over all tasks, the most elementary 24-hours linear bar chart
is most accurate and efficient and is also preferred by the users. This
provides strong evidence for the use of linear layouts - even for visualizing
periodical daily patterns.
