selection_interval does not work with fields

[krlng]

The selection_interval seems to work neither with fields nor with encodings which are not numeric. Is this a bug or is there simply no support?

Example code:

import altair as alt
from vega_datasets import data as vdata

cars = vdata.cars.url

brush = alt.selection_interval(fields=['Name'])

alt.Chart(cars).mark_point().encode(
    x='Miles_per_Gallon:Q',
    y='Horsepower:Q',
    color='Origin:N',
    tooltip=["Name:N"]
).add_selection(
    brush
)

error message: Expression parse error: ? selector072_scale_trigger : {}

The same happens if I use brush = alt.selection_interval(encodings=['color'])

[jakevdp]

This is expected: in Vega-Lite, interval selections only work with x and y encodings, and not with fields.

[krlng]

Okay, kind of sad because that would have been really cool, but well..