Version 3.0 wish list

[jakevdp]

It's mostly documentation at this point:

• update DOC references to low-level classes (DOC: update sphinx docs for Altair 3 #1397)
• add new transforms to transform docs (Add transform methods for Vega-Lite 3 transforms #1387, DOC: add documentation of flatten transform #1439, DOC: add narrative docs for fold transform #1440, DOC: add narrative docs of impute transform #1441, DOC: add docs for joinaggregate transform #1442, DOC: add docs for sample transform #1446)
• update fold transform example (DOC: update parallel coordinates chart #1390)
• add examples of new composite marks (DOC: use boxplot mark for boxplot example #1389, ENH: Add Error Bars with Standard Deviation example #1396, DOC: add CI error band example #1398)
• add documentation of new composite marks
• add examples of new encoding channels (DOC: update sphinx docs for Altair 3 #1397, DOC: add all encodings to tables #1401)
• add documentation of new encoding channels
• add documentation of default tooltip behavior & how to disable
• add documentation & examples of wrapped facet (example of wrapped facet #1410)
• add documentation & examples of wrapped concat
• add documentation & examples of wrapped repeat
• add examples of initializing selections (DOC: add initialization to binding examples #1413)
• Bug Save Chart option with transformer behavior is wrong #1370: respect transformers in HTML export
• Bug Documentation bug: expr.format's second, not first argument, is the format specifier #1411: fix docs for alt.expr functions (implicit: also update wrapped functions for VL3 additions)
• Add specific errors for common gotchas #1375 Improve error messages for common gotchas (need to check whether these all still apply in Vega-Lite 3)