  Annotations play a vital role in highlighting critical aspects of
visualizations, aiding in data externalization and exploration, collaborative
sensemaking, and visual storytelling. However, despite their widespread use, we
identified a lack of a design space for common practices for annotations. In
this paper, we evaluated over 1,800 static annotated charts to understand how
people annotate visualizations in practice. Through qualitative coding of these
diverse real-world annotated charts, we explored three primary aspects of
annotation usage patterns: analytic purposes for chart annotations (e.g.,
present, identify, summarize, or compare data features), mechanisms for chart
annotations (e.g., types and combinations of annotations used, frequency of
different annotation types across chart types, etc.), and the data source used
to generate the annotations. We then synthesized our findings into a design
space of annotations, highlighting key design choices for chart annotations. We
presented three case studies illustrating our design space as a practical
framework for chart annotations to enhance the communication of visualization
insights. All supplemental materials are available at
{https://shorturl.at/bAGM1}.
