Altair selections resolve incorrectly as scale domain values.

I am observing this issue in Altair 2.x. Apologies if this issue has already been addressed for v3!

The following code for an overview + detail plot fails:

brush = alt.selection_interval(encodings=['x']);

base = alt.Chart().mark_area().encode(
    x=alt.X('date:T', title=None),
    y='price:Q'
).properties(
    width=700
)
  
alt.vconcat(
    base.encode(alt.X('date:T', title=None, scale=alt.Scale(domain=brush))),
    base.add_selection(brush).properties(height=60),
    data='https://vega.github.io/vega-datasets/data/sp500.csv'
)

The errors indicate that Altair is expanding the brush to the full selection definition (what you would use for add_selection) rather than generating a selection reference. If I add name='brush' to the selection definition and then replace domain=brush with domain={'selection': 'brush'}, the example works.

[jakevdp]

One way we could maybe improve this is to not make Altair selections equivalent Vega-Lite selection objects, but rather make them objects with a JSON representation of {selection: "name"} , but which resolve the same way as selection objects within the context of add_selection. It would take a bit of refactoring, but probably would be better.

[jakevdp]

See #1433