Vega:

• Added a transform to impute missing values.
• Updated the Bin transform to automatically calculate the minimum and maximum bin values if none are specified. The end binned value is also added to each tuple. For consistency, this release introduces a breaking change: the former bin output property has been renamed to bin_start.
• Production rules can now be specified directly as an array, rather than as a nested rule property within a visual property. The latter functionality is now deprecated, and will be removed in a future release.
• vg.Transform, vg.BatchTransform, and vg.Parameter classes have been exposed, to make it easier to build custom 3rd-party transformations.
• Individual transformation schemas are now addressable, e.g. "$ref": "#/defs/aggregateTransform".
• Fixed bug with the scene graph data join using old changesets from datasources (#387).
• Fixed bug with signal propagation prematurely ending (#386).
• Fixed bug with the Facet transform not transmitting changeset metadata to its cells' datasources.
• Fixed bug with inverting ordinal scales with ranges sorted in descending order.
• Fixed bug to correctly enumerate the Stack transform's offset property.
• Updated to the latest version of d3-cloud, to remove the dependency on node-canvas.

Vega Dataflow:

• Fixed bug with derived datasources not applying upstream sort functions.
• Friendlier error messages if the system encounters unknown signals or data sources references in a specification.

Vega Scenegraph:

• Fixed bugs with group mark rendering, clipping, and hit testing.

Vega Editor:

• New examples of a horizon chart and reordering a matrix.

Fixes a bug with vega-dataflow that incorrectly threw an error message with multiple interdependent datasources (#368).

• Adds a filter parameter to the Cross transform (kudos @Jerrythafast)
• Ordinal scale domains can be sorted in natural/key order in addition to an aggregate measure.
• The sort.order property for ordinal scale domains has been deprecated in favor of the reverse property on the scale object.
• For quantitative scales, domainMin and domainMax only override domain if they are valid (i.e., not null, undefined, NaN, etc.).
• Adds D3's category20b and category20c as valid scale range literals.
• Fixes a bug with the data-join where new streaming tuples failed to key match an existing scene graph item.

This release adds a useful method to the View class: toImageURL for exporting image data (png or svg) from a Vega visualization.

view.toImageURL('png'); // returns a dataURL of PNG image data
view.toImageURL('svg'); // returns a dataURL of SVG image data

The returned URL can be opened in a new browser window or (on supporting browsers) made into a downloadable link via an a tag with href set to the provided URL and the download attribute set to the desired file name.

Version 2.2 introduces major refinements to internal data management, resulting in significant improvements to both initialization time and interactive performance.

Vega:

• Migrate to vega-dataflow 1.2.
• Migrate to datalib 1.4.5 aggregator.
• Optimize streaming aggregation updates.
• Improve default axis formatting for log scales.
• Change transform reset parameter from flag to counter.
• Fix force layout parameter update bug.
• Remove deprecated Zip transform. (Use Lookup instead.)

Vega Dataflow:

• Overhaul tuple derivation and previous value handling.
• Remove now unneeded dataflow node "revises" flag.

Vega:

• Add CountPattern transform.
• Facet transform no longer requires a "groupby" directive.
• Add interactivity to Force transform.
• Add Lookup transform.
• Add Wordcloud transform.
• Add Voronoi transform.
• Deprecate Zip transform. (Use Lookup instead.)
• Add locale formatting support (via datalib 1.4.3)
• Add "open" expression function for hyperlinking.
• Refine schema and test cases.
• Miscellaneous bug fixes.

Vega Editor:

• Add airports, force_drag and wordcloud examples.
• Modify choropleth and force examples to use lookup transform.

• Declarative interaction design using Signals and Predicates.
• Support for streaming data sets.
• A "reactive geometry" which allows generated mark instances to be used as the backing data for downstream marks.
• Overhauled scenegraph construction, rendering, and event handling with the vega-scenegraph module:
  • Parity between Canvas and SVG modes.
  • Improved performance of SVG renderer.
  • Support for incremental re-rendering.
• Improved editor (now found in the vega-editor repository) with syntax highlighting, code folding, and error icons for JSON parse errors.
• JSON schema to validate Vega specifications against.
• More expressive aggregate and facet data transforms.
• Major improvements to code quality via jshint and extensive unit tests.