  Increased access to mobile devices motivates the need to design communicative
visualizations that are responsive to varying screen sizes. However, relatively
little design guidance or tooling is currently available to authors. We
contribute a detailed characterization of responsive visualization strategies
in communication-oriented visualizations, identifying 76 total strategies by
analyzing 378 pairs of large screen (LS) and small screen (SS) visualizations
from online articles and reports. Our analysis distinguishes between the
Targets of responsive visualization, referring to what elements of a design are
changed and Actions representing how targets are changed. We identify key
trade-offs related to authors' need to maintain graphical density, referring to
the amount of information per pixel, while also maintaining the "message" or
intended takeaways for users of a visualization. We discuss implications of our
findings for future visualization tool design to support responsive
transformation of visualization designs, including requirements for automated
recommenders for communication-oriented responsive visualizations.
