Update to Vega-Lite 3.2 and Vega 5.3 & support all new features. See
https://github.com/vega/vega-lite/releases/tag/v3.0.0 for Vega-Lite
feature lists.

Highlights

new compound marks: mark_boxplot(), mark_errorband(), mark_errorbar()
new transforms: transform_impute(), transform_joinaggregate(), transform_flatten() transform_fold(), transform_sample(), transform_stack()
new facet encoding that is similar to the row and column encoding, but allows for wrapped facets
new alt.concat() function that is similar to alt.hconcat and alt.vconcat, but allows for more general wrapped concatenation
new columns keyword that allows wrapped faceting, repeating, and concatenation.
many, many bug fixes
tooltips can now be automatically populated using the tooltip mark configuration.
ability to specify initial conditions for selections

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