  Timelines are commonly represented on a horizontal line, which is not
necessarily the most effective way to visualize temporal event sequences.
However, few experiments have evaluated how timeline shape influences task
performance. We present the design and results of a controlled experiment run
on Amazon Mechanical Turk (n=192) in which we evaluate how timeline shape
affects task completion time, correctness, and user preference. We tested 12
combinations of 4 shapes -- horizontal line, vertical line, circle, and spiral
-- and 3 data types -- recurrent, non-recurrent, and mixed event sequences. We
found good evidence that timeline shape meaningfully affects user task
completion time but not correctness and that users have a strong shape
preference. Building on our results, we present design guidelines for creating
effective timeline visualizations based on user task and data types. A free
copy of this paper, the evaluation stimuli and data, and code are available at
https://osf.io/qr5yu/
