  Creating an animated data video enriched with audio narration takes a
significant amount of time and effort and requires expertise. Users not only
need to design complex animations, but also turn written text scripts into
audio narrations and synchronize visual changes with the narrations. This paper
presents WonderFlow, an interactive authoring tool, that facilitates
narration-centric design of animated data videos. WonderFlow allows authors to
easily specify a semantic link between text and the corresponding chart
elements. Then it automatically generates audio narration by leveraging
text-to-speech techniques and aligns the narration with an animation.
WonderFlow provides a visualization structure-aware animation library designed
to ease chart animation creation, enabling authors to apply pre-designed
animation effects to common visualization components. It also allows authors to
preview and iteratively refine their data videos in a unified system, without
having to switch between different creation tools. To evaluate WonderFlow's
effectiveness and usability, we created an example gallery and conducted a user
study and expert interviews. The results demonstrated that WonderFlow is easy
to use and simplifies the creation of data videos with narration-animation
interplay.
