Tracking: complete the Animated Vega-Lite implementation

[jonathanzong]

#8921 landed the basics of Animated Vega-Lite: a time encoding, a timer point selection, and a frame filter. It deferred interpolation, rescaling, interactive sliders, and data-driven pausing, and hard-errored on multi-view specs.

This issue tracks finishing that work. The implementation splits into small additive PRs rather than one large PR, layered on top of the animation PRs already in flight.

Related: #4060. Closes #9590 (linear and interpolated animations) once #9910 and #9914 land.

Already in flight

One has merged; two remain open, and everything else sits on top of them.

• fix(selection): initialize animation selections on the first pulse #9900 — fix(selection): initialize animation selections on the first pulse
  Merged as 6fd39f971. Compiles an animation's tuple and modify signals as update expressions rather than on handlers, so the store fills on the first pulse instead of one tick late. A headless render now lands on the first frame rather than drawing every frame at once.
• fix: support animated vega-lite thumbnail rendering with vg2png and vg2svg vega#4061 — fix: support animated vega-lite thumbnail rendering with vg2png and vg2svg
  Evaluates the dataflow and clears animation timers before rendering, so headless renders are deterministic. Unblocked now that fix(selection): initialize animation selections on the first pulse #9900 has merged.
• docs: animated time encoding documentation and examples #9535 — docs: animated time encoding documentation and examples
  Documents the time channel and registers the existing animated examples in the gallery. Depends on fix: support animated vega-lite thumbnail rendering with vg2png and vg2svg vega#4061 for deterministic thumbnails.

Independent

Reviewable in any order, with no dependency on the animation stack or on each other.

• A. feat(vega-functions): add easing functions and interpolateLinear vega#4316 — feat(vega-functions): add easing functions and interpolateLinear
  Adds d3-ease functions and interpolateLinear to the expression language. Only feat(animation): easing for animated selections #9916 needs it.
• B. fix(layer): do not emit duplicate signals for a layer-level selection #9901 — fix(layer): do not emit duplicate signals for a layer-level selection
  Fixes a pre-existing bug on main, unrelated to animation. A selection applied to more than one child of a layer produces duplicate signals and an unparseable spec. 2 files. feat(animation): support animation in layered and concatenated views #9915 needs it.
• C. feat(selection): field-comparison predicates for point selections #9902 — feat(selection): field-comparison predicates for point selections
  Adds predicate, which makes a point selection hold a comparison rather than a value. The feature serves ordinary interactions too, and animation depends on it for frames that keep everything up to the clock or a trailing window of it, and for scale binding. Fully self-contained: its example is selection_predicate_cumulative, and the bird migration example that needs it lives in feat(animation): support animation in layered and concatenated views #9915, which adds the layer support its basemap needs.
• M. feat: add bird migration centroids vega-datasets#797 — feat: add bird migration centroids
  Adds dataset of daily population centroids for transatlantic migratory bird species. Needed for the bird migration example in feat(animation): support animation in layered and concatenated views #9915, so M needs to be released before that one merges.

Animation stack — merge in order

Every PR targets main, and states its dependencies in its own description rather than encoding them as a base branch. In git each branch still descends from the one before it, and all of them descend from #9535, whose documentation and source changes the stack edits. GitHub therefore shows a cumulative diff: PR #9916 contains every commit from #9535 through #9915 as well as its own.

Landing B and C early pays off: B first removes #9915's cherry-pick, and C first is required to pass CI for every branch #9910 onward.

• D. fix(animation): build the animation frame dataset reliably #9903 — fix(animation): build the animation frame dataset reliably
  Fixes the bugs in how the per-frame dataset is built and read: where the frame filter is found, when the dataset exists, which marks read it, how its rows are laid out, and how its time field parses.
• E. feat(animation): elaborate a time encoding into an animation parameter #9909 — feat(animation): elaborate a time encoding into an animation parameter
  A time encoding now animates on its own, and the normalizer derives the timer parameter, the frame filter, and the projection onto the time field.
• F. feat(animation): support linear time scales #9910 — feat(animation): support linear time scales
  Adds continuous playback, makes {"step": ...} expressible in a spec, and restores animationDuration, which docs: animated time encoding documentation and examples #9535 comments out pending this work. Introduces the earthquake example, which feat(animation): support animation in layered and concatenated views #9915 later gives a basemap.
• G. feat(animation): recompute scales per frame with time.rescale #9911 — feat(animation): recompute scales per frame with time.rescale
  Enables updating scales based on the current frame, required for e.g. bar chart races.
• H. feat(animation): support pausing playback #9912 — feat(animation): support pausing playback
• I. feat(animation): bind an animated selection to a range input #9913 — feat(animation): bind an animated selection to a range input
• J. feat(animation): interpolate marks between keyframes with time.key #9914 — feat(animation): interpolate marks between keyframes with time.key
  The largest change in the stack. Enables keying so that marks representing the same object interpolate across frames. Also reveals animated line marks from their full geometry, so curve interpolation stays stable as a line extends.
• K. feat(animation): support animation in layered and concatenated views #9915 — feat(animation): support animation in layered and concatenated views
  Introduces the bird migration example (animated points over a basemap layer), so its images depend on M.
• L. feat(animation): easing for animated selections #9916 — feat(animation): easing for animated selections
  Requires A to be released. Only PR in the stack that needs a new Vega.
• N. feat(animation): support animation in faceted views #9917 — feat(animation): support animation in faceted views
  Builds each cell's frame dataset inside its group, which covers the last kind of multi-view composition.
• O. docs(examples): register the animated examples in the gallery #9918 — docs(examples): register the animated examples in the gallery
  Curation only, one file. Merges last, because it registers examples that feat(selection): field-comparison predicates for point selections #9902, feat(animation): support linear time scales #9910, feat(animation): recompute scales per frame with time.rescale #9911, feat(animation): bind an animated selection to a range input #9913, feat(animation): interpolate marks between keyframes with time.key #9914, and feat(animation): support animation in layered and concatenated views #9915 each add. Registering a spec whose PR has not landed breaks scripts/create-example-pages, which reads every registered spec off disk.

[domoritz]

Amazing progress on this. I have a few things on my plate this week but will try to help get this over the finish line soon.