  Charts often contain visually prominent features that draw attention to
aspects of the data and include text captions that emphasize aspects of the
data. Through a crowdsourced study, we explore how readers gather takeaways
when considering charts and captions together. We first ask participants to
mark visually prominent regions in a set of line charts. We then generate text
captions based on the prominent features and ask participants to report their
takeaways after observing chart-caption pairs. We find that when both the chart
and caption describe a high-prominence feature, readers treat the doubly
emphasized high-prominence feature as the takeaway; when the caption describes
a low-prominence chart feature, readers rely on the chart and report a
higher-prominence feature as the takeaway. We also find that external
information that provides context, helps further convey the caption's message
to the reader. We use these findings to provide guidelines for authoring
effective chart-caption pairs.
