Altair

- Update Vega-Lite from version 4.8.1 to version 4.17.0; see [Vega-Lite Release Notes](https://github.com/vega/vega-lite/releases).

Enhancements
- Pie charts are now supported through the use of `mark_arc`. (Examples: eg. [Pie Chart](https://altair-viz.github.io/gallery/pie_chart.html#gallery-pie-chart) and [Radial Chart](https://altair-viz.github.io/gallery/radial_chart.html#gallery-radial-chart))
- Support for the `datum` encoding specifications from Vega-Lite; see [Vega-Lite Datum Definition](https://vega.github.io/vega-lite/docs/encoding.html#datum-def). (Examples: [Line Chart with datum](https://altair-viz.github.io/gallery/line_chart_with_datum.html#gallery-line-chart-with-datum) and [Line Chart with datum for color](https://altair-viz.github.io/gallery/line_chart_with_color_datum.html#gallery-line-chart-with-color-datum).)
- ``angle`` encoding can now be used to control point styles (Example: [Wind Vector Map](https://altair-viz.github.io/gallery/wind_vector_map.html#gallery-wind-vector-map))
- Support for serialising pandas nullable data types for float data (2399).
- Automatically create an empty data object when `Chart` is called without a data parameter (2515).
- Allow the use of pathlib Paths when saving charts (2355).
- Support deepcopy for charts (2403).

Bug Fixes
- Fix `to_dict()` for nested selections (2120).
- Fix item access for expressions (2099).

- Minimum Python version is now 3.6
- Update Vega-Lite to version 4.8.1; many new features and bug fixes from Vega-Lite versions 4.1 through 4.8; see [Vega-Lite Release Notes](https://github.com/vega/vega-lite/releases).

Enhancements

- ``strokeDash`` encoding can now be used to control line styles (Example: [Multi Series Line Chart](https://altair-viz.github.io/gallery/multi_series_line.html)
- ``chart.save()`` now relies on [altair_saver](http://github.com/altair-viz/altair_saver) for more flexibility (#1943).
- New ``chart.show()`` method replaces ``chart.serve()``, and relies on [altair_viewer](http://github.com/altair-viz/altair_viewer) to allow offline viewing of charts (#1988).

Bug Fixes

- Support Python 3.8 (1958)
- Support multiple views in JupyterLab (1986)
- Support numpy types within specifications (1914)
- Support pandas nullable ints and string types (1924)

Maintenance

- Altair now uses [black](https://github.com/psf/black) and [flake8](https://gitlab.com/pycqa/flake8) for maintaining code quality & consistency.

Bug fixes
- Update Vega-Lite version to 4.0.2
- Fix issue with duplicate chart divs in HTML renderer (1888)

https://github.com/vega/vega-lite/releases/tag/v4.0.0.

It is the first version of Altair to drop Python 2 compatibility, and is tested
on Python 3.5 and newer.

Enhancements

- Support for interactive legends: ([Example](http://altair-viz.github.io/gallery/interactive_legend.html#gallery-interactive-legend))
<img height="200" alt="interactive legend" src="https://user-images.githubusercontent.com/781659/70592444-aec9fb00-1b8e-11ea-8bf7-199a16810e0d.gif">

- Responsive chart width and height: ([Example](http://altair-viz.github.io/user_guide/customization.html#customization-chart-size))
<img height="200" alt="dynamic width" src="https://user-images.githubusercontent.com/781659/70593197-f782b380-1b90-11ea-8dcf-29282ea913b0.gif">

- Bins responsive to selections: ([Example](http://altair-viz.github.io/gallery/histogram_responsive.html#gallery-histogram-responsive))
<img height="200" alt="responsive bin" src="https://user-images.githubusercontent.com/781659/70596199-19346880-1b9a-11ea-9dec-f9739c2b055a.gif">

- New pivot transform: ([Example](http://altair-viz.github.io/user_guide/transform/pivot.html#user-guide-pivot-transform))
<img height="200" alt="pivot" src="https://user-images.githubusercontent.com/781659/70593369-a4f5c700-1b91-11ea-8655-9af0fbb3c149.gif">

- New Regression transform: ([Example](http://altair-viz.github.io/user_guide/transform/regression.html#user-guide-regression-transform))
<img height="200" alt="regression" src="https://user-images.githubusercontent.com/781659/70592667-78d94680-1b8f-11ea-999e-4b37725793a8.png">

- New LOESS transform: ([Example](http://altair-viz.github.io/user_guide/transform/loess.html#user-guide-loess-transform))
<img height="200" alt="loess" src="https://user-images.githubusercontent.com/781659/70592674-80005480-1b8f-11ea-99bf-9a36d5a50bd3.png">

- New density transform: ([Example](http://altair-viz.github.io/user_guide/transform/density.html#user-guide-density-transform))
<img height="200" alt="density" src="https://user-images.githubusercontent.com/781659/70592687-87bff900-1b8f-11ea-842b-0a68f666e26b.png">

- Image mark ([Example](http://altair-viz.github.io/user_guide/marks.html#image-mark))
<img height="200" alt="image" src="https://user-images.githubusercontent.com/781659/70592972-4b40cd00-1b90-11ea-93ca-81d49c196053.png">


- New default ``html`` renderer, directly compatible with Jupyter Notebook and
JupyterLab without the need for frontend extensions, as well as tools like
nbviewer and nbconvert, and related notebook environments such as Zeppelin,
Colab, Kaggle Kernels, and DataBricks. To enable the old default renderer, use:

alt.renderers.enable('mimetype')


- Support per-corner radius for bar marks: ([Example](http://altair-viz.github.io/gallery/bar_rounded.html#gallery-bar-rounded))
<img height="300" alt="round-bar" src="https://user-images.githubusercontent.com/781659/70592714-99090580-1b8f-11ea-9304-03d3967d600c.png">

Grammar Changes
- Sort-by-field can now use the encoding name directly. So instead of

alt.Y('y:Q', sort=alt.EncodingSortField('x_field', order='descending'))

you can now use::

alt.Y('y:Q', sort="-x")


- The ``rangeStep`` argument to :class:`Scale` and :meth:`Chart.configure_scale` is deprecated.
instead, use ``chart.properties(width={"step": rangeStep})`` or
``chart.configure_view(step=rangeStep)``.

- ``align``, ``center``, ``spacing``, and ``columns`` are no longer valid chart properties, but
are moved to the encoding classes to which they refer.

*released Nov 27, 2019*

Last release to support Python 2

Enhancements

- Add inheritance structure to low-level schema classes (1803)
- Add ``html`` renderer which works across frontends (1793)
- Support Python 3.8 (1740, 1781)
- Add ``:G`` shorthand for geojson type (1714)
- Add data generator interface: ``alt.sequence``, ``alt.graticule``,
``alt.sphere()`` (1667, 1687)
- Support geographic data sources via ``__geo_interface__`` (1664)

Bug Fixes

- Support ``pickle`` and ``copy.deepcopy`` for chart objects (1805)
- Fix bug when specifying ``count()`` within ``transform_joinaggregate()`` (1751)
- Fix ``LayerChart.add_selection`` (1794)
- Fix arguments to ``project()`` method (1717)
- Fix composition of multiple selections (1707)

Upgraded to Vega-Lite version 3.4 (See [Vega-Lite 3.4 Release Notes](https://github.com/vega/vega-lite/releases/tag/v3.4.0)).

Following are changes to Altair in addition to those that came with VL 3.4:

Enhancements

- Selector values can be used directly in expressions (1599)
- Top-level chart repr is now truncated to improve readability of error
messages (1572)

Bug Fixes

- top-level ``add_selection`` methods now delegate to sub-charts. Previously
they produced invalid charts (1607)
- Unsupported ``mark_*()`` methods removed from LayerChart (1607)
- New encoding channels are properly parsed (1597)
- Data context is propagated when encodings are specified as lists (1587)

Backward-Incompatible Changes
- ``alt.LayerChart`` no longer has ``mark_*()`` methods, because they never
produced valid chart specifications) (1607)