  Establishing common ground and maintaining shared awareness amongst
participants is a key challenge in collaborative visualization. For real-time
collaboration, existing work has primarily focused on synchronizing constituent
visualizations - an approach that makes it difficult for users to work
independently, or selectively attend to their collaborators' activity. To
address this gap, we introduce a design space for representing synchronous
multi-user collaboration in visualizations defined by two orthogonal axes:
situatedness, or whether collaborators' interactions are overlaid on or shown
outside of a user's view, and specificity, or whether collaborators are
depicted through abstract, generic representations or through specific means
customized for the given visualization. We populate this design space with a
variety of examples including generic and custom synchronized cursors, and user
legends that collect these cursors together or reproduce collaborators' views
as thumbnails. To build common ground, users can interact with these
representations by peeking to take a quick look at a collaborator's view,
tracking to follow along with a collaborator in real-time, and forking to
independently explore the visualization based on a collaborator's work. We
present a reference implementation of a wrapper library that converts
interactive Vega-Lite charts into collaborative visualizations. We find that
our approach affords synchronous collaboration across an expressive range of
visual designs and interaction techniques.
