Adds vega altair style example for the gallery.

[MarcSkovMadsen]

Addresses #3033

Makes it easier for users to figure out how they can style their Altair and Vega plots

You can try out the notebook and app on binder

• notebook: https://mybinder.org/v2/gh/holoviz/panel/altair-style?urlpath=lab/tree/examples/gallery/styles/VegaAltairStyle.ipynb
• app: https://mybinder.org/v2/gh/holoviz/panel/altair-style?urlpath=panel/VegaAltairStyle

Additional Context

• Implementing this one would make the example simpler Make it easy to determine the current theme and to change it dynamically #2952.

Todo:

• [] Philipp to implement pn.state.theme
• [] Marc to update notebook to use pn.state.theme.

Promo Video and Gif

VegaAltairStyle.mp4

[philippjfr]

So I really can't merge any of these PRs because of the large gifs in them. As I suggested on the Home app PR could you send me all the gifs you have as a zip file (via email if necessary) so I can upload them all at once. The only other comment I have is about the get_theme() method and parsing the session_args. I will make a separate PR to expose pn.state.theme explicitly, which should simplify quite a lot of that.

[MarcSkovMadsen]

For now I will include the link to this gif. Please use this link instead @philippjfr

[codecov]

Codecov Report

Merging #3032 (06b1b48) into master (09bab95) will decrease coverage by 0.26%.
The diff coverage is 0.00%.

❗ Current head 06b1b48 differs from pull request most recent head 33f280b. Consider uploading reports for the commit 33f280b to get more accurate results

@@            Coverage Diff             @@
##           master    #3032      +/-   ##
==========================================
- Coverage   83.25%   82.99%   -0.27%     
==========================================
  Files         190      193       +3     
  Lines       24971    25481     +510     
==========================================
+ Hits        20789    21147     +358     
- Misses       4182     4334     +152     

Impacted Files	Coverage Δ
panel/script.py	0.00% <0.00%> (ø)
panel/models/tabulator.py	95.45% <0.00%> (-3.18%)	⬇️
panel/io/server.py	71.94% <0.00%> (-1.70%)	⬇️
panel/tests/conftest.py	94.85% <0.00%> (-1.05%)	⬇️
panel/auth.py	39.94% <0.00%> (-0.68%)	⬇️
panel/widgets/tables.py	84.97% <0.00%> (-0.58%)	⬇️
panel/command/serve.py	39.26% <0.00%> (-0.31%)	⬇️
panel/pane/vega.py	83.04% <0.00%> (-0.30%)	⬇️
panel/template/base.py	78.43% <0.00%> (-0.28%)	⬇️
panel/tests/test_pipeline.py	98.92% <0.00%> (-0.02%)	⬇️
... and 33 more

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[philippjfr]

Here's the link: https://assets.holoviews.org/panel/thumbnails/gallery/styles/vega-styles.gif