Lifting parameters to the top level

[ChristopherDavisUCI]

This should have the same functionality as #2684, just with a cleaner commit history. The only change in functionality from #2684 is that layer is allowed to be mixed with row or column in Repeat Charts.

The main goal of this PR is to improve the behavior of parameters in multi-view charts (whether added at the top level or at the level of the constituent charts).

Thanks to @mattijn especially for the dozens of comments and examples.

[ChristopherDavisUCI]

The following example should be working on this branch.

import altair as alt
from vega_datasets import data

source = data.movies.url

alt.Chart(source).mark_line().encode(
    x=alt.X(alt.repeat("column"), bin=True),
    # x=alt.X("IMDB_Rating:Q", bin=True),
    y=alt.Y(
        alt.repeat("layer"), aggregate="mean", title="Mean of US and Worldwide Gross"
    ),
    color=alt.ColorDatum(alt.repeat("layer")),
).repeat(
    layer=["US_Gross", "Worldwide_Gross"],
    column=["IMDB_Rating", "Rotten_Tomatoes_Rating"],
)

[ChristopherDavisUCI]

@jakevdp @joelostblom @mattijn I think what's here is pretty much finalized. Are there any changes you'd like to see? For now I'm planning to switch focus to #2592 (comment)

Thanks again @mattijn for all the help.

[ChristopherDavisUCI]

I added the changes from #2591 to this PR. Thanks @joelostblom for noticing #2591 had been mistakenly deleted!

[mattijn]

Thanks! All tests on Github Actions are passing.
The VL-specification are created like we anticipated for the compound charts. Still a few outstanding issues for rendering on the VL-side. Just listing for reference:

• repeat operator with defined width and height does not accept layer vega-lite#8523
• repeat chart with top level defined brush selection corrupts cross-view interaction vega-lite#8348
• chart with repeat and a defined name disables the independency of the repeated channel (eg y-axis) vega-lite#8446
• duplicate parameter definition error in layered charts (because of "point": true) vega-lite#7854

Merging this🥳

[joelostblom]

Awesome seeing this merged, thank you both for your hard work on it!