  Animated transitions help viewers follow changes between related
visualizations. Specifying effective animations demands significant effort:
authors must select the elements and properties to animate, provide transition
parameters, and coordinate the timing of stages. To facilitate this process, we
present Gemini, a declarative grammar and recommendation system for animated
transitions between single-view statistical graphics. Gemini specifications
define transition "steps" in terms of high-level visual components (marks,
axes, legends) and composition rules to synchronize and concatenate steps. With
this grammar, Gemini can recommend animation designs to augment and accelerate
designers' work. Gemini enumerates staged animation designs for given start and
end states, and ranks those designs using a cost function informed by prior
perceptual studies. To evaluate Gemini, we conduct both a formative study on
Mechanical Turk to assess and tune our ranking function, and a summative study
in which 8 experienced visualization developers implement animations in D3 that
we then compare to Gemini's suggestions. We find that most designs (9/11) are
exactly replicable in Gemini, with many (8/11) achievable via edits to
suggestions, and that Gemini suggestions avoid multiple participant errors.
