ordering of legend labels

Hi Jake,

Can we change around the order of the legend entries?

Say, put Google at the top in this example from the docs:

import altair as alt
from vega_datasets import data

stocks = data.stocks()

alt.Chart(stocks).mark_line().encode(
    x='date',
    y='price',
    color='symbol'
)

[jakevdp]

You can specify the order using the sort property of the channel. Categories will appear in the specified order, and any remaining categories will be appended in arbitrary order. So if all you care is that GOOG is first, you can do this:

import altair as alt
from vega_datasets import data

stocks = data.stocks()

alt.Chart(stocks).mark_line().encode(
    x='date',
    y='price',
    color=alt.Color('symbol', sort=['GOOG'])
)

[jakevdp]

This should be fixed as of Altair 4.0