  We present a systematic review and design space for visualizations on
smartwatches and the context in which these visualizations are
displayed--smartwatch faces. A smartwatch face is the main smartwatch screen
that wearers see when checking the time. Smartwatch faces are small data
dashboards that present a variety of data to wearers in a compact form. Yet,
the usage context and form factor of smartwatch faces pose unique design
challenges for visualization. In this paper, we present an in-depth review and
analysis of visualization designs for popular premium smartwatch faces based on
their design styles, amount and types of data, as well as visualization styles
and encodings they included. From our analysis we derive a design space to
provide an overview of the important considerations for new data displays for
smartwatch faces and other small displays. Our design space can also serve as
inspiration for design choices and grounding of empirical work on smartwatch
visualization design. We end with a research agenda that points to open
opportunities in this nascent research direction. Supplementary material is
available at: https://osf.io/nwy2r/.
