  Captions help readers better understand visualizations. However, if the
visualization is intended to communicate specific features, should the caption
be statistical, and focus on specific values, or perceptual, and focus on
general patterns? Prior work has shown that when captions mention visually
salient features, readers tend to recall those features. Still, we lack
explicit guidelines for how to compose the appropriate caption. Further, what
if the author wishes to emphasize a less salient feature?
  In this paper, we study how the visual salience of the feature described in a
caption, and the semantic level of the caption description, affect a reader's
takeaways from line charts. For each single- or multi-line chart, we generate 4
captions that 1) describe either the primary or secondary salient feature in a
chart, and 2) describe the feature either at the statistical or perceptual
levels. We then show participants random chart-caption pairs and record their
takeaways.
  We find that the primary salient feature is more memorable for single-line
charts when the caption is expressed at the statistical level; for primary and
secondary features in multi-line charts, the perceptual level is more
memorable. We also find that many readers will tend to recall y-axis numerical
values when a caption is present.
