Is it possible to transform_filter with a test against a list?

Here’s what I mean.

import altair as alt
from vega_datasets import data
df = data.stocks()
alt.Chart(df).mark_line().encode(
    x='date:T',
    y='price:Q',
    color="symbol",
    column="symbol:N"
).transform_filter(alt.datum.symbol in ["AAPL", "AMZN"])

I’m trying to spit out a restricted list of two charts, for Apple and Amazon. But this raises an error.

SchemaValidationError: Invalid specification

        altair.vegalite.v2.schema.core.FilterTransform ->filter, validating 'anyOf'

        True is not valid under any of the given schemas

This is a use case I see as pretty common. For instance, after grouping on a field I’d like to spit out mini multiple charts of the top ten ranking values.

[jakevdp]

Oh, I should have also mentioned that a wrapper for this exists in Altair:

transform_filter(alt.FieldOneOfPredicate(field='symbol', oneOf=['AAPL', 'AMZN']))

[jakevdp]

It would not be possible to make this work using something like alt.datum.symbol in ["AAPL", "AMZN"], because in Python x in y always returns a boolean, and the language provides no mechanism to overload the behavior of the in operator.