  Animated visualization is becoming increasingly popular as a compelling way
to illustrate changes in time series data. However, maintaining the viewer's
focus throughout the entire animation is difficult because of its
time-consuming nature. Viewers are likely to become bored and distracted during
the ever-changing animated visualization. Informed by the role of foreshadowing
that builds the expectation in film and literature, we introduce visual
foreshadowing to improve the engagement of animated visualizations. In
specific, we propose designs of visual foreshadowing that engage the audience
while watching the animation. To demonstrate our approach, we built a
proof-of-concept animated visualization authoring tool that incorporates visual
foreshadowing techniques with various styles. Our user study indicates the
effectiveness of our foreshadowing techniques on improving engagement for
animated visualization.
