  Narrative visualization is a powerful communicative tool that can take on
various formats such as interactive articles, slideshows, and data videos.
These formats each have their strengths and weaknesses, but existing authoring
tools only support one output target. We conducted a series of formative
interviews with seven domain experts to understand needs and practices around
cross-format data stories, and developed Fidyll, a cross-format compiler for
authoring interactive data stories and explorable explanations. Our open-source
tool can be used to rapidly create formats including static articles,
low-motion articles, interactive articles, slideshows, and videos. We evaluate
our system through a series of real-world usage scenarios, showing how it
benefits authors in the domains of data journalism, scientific publishing, and
nonprofit advocacy. We show how Fidyll, provides expressive leverage by
reducing the amount of non-narrative markup that authors need to write by
80-90% compared to Idyll, an existing markup language for authoring interactive
articles.
