alt.Text(' ') not allowed anymore?

The Tutorial `10-HeatMaps.ipynb’ contains as last example a call where the cell text is replaced by an empty string.

This errors on Python 3.6 with GH master of altair using JupyterLab/GitHub proxy extension:

alt.Chart(cars).mark_text(applyColorToBackground=True).encode(
    alt.Row('Origin:O'),
    alt.Column('Cylinders:O'),
    alt.Color('mean(Miles_per_Gallon):Q', sort='descending', scale=alt.Scale(range=['pink', 'red'])),
    alt.Text(' '),
)

FieldError: Fields/columns not found in the data: {' '}

[jakevdp]

The problem is that most errors that come up will not be python errors, but javascript rendering errors. Our introduction of FieldError in the most recent release was an attempt to move some of the (silent) javascript errors into more noticeable python-side errors; given all the problems it’s caused I think we should probably revert that change. But that will move these types of errors back to the javascript side, so CI tests won’t catch them.

[jakevdp]

This is obsolete now that we’ve moved to v2.0