altair.data_transformers.enable('json') leads to invalid data URLs in JupyterLab when switching from Altair 3.3.0 to 4.0.0

To reproduce, use the latest version (as of this writing) of altair and jupyterlab:

altair==4.0.0
ipykernel==5.1.3
ipython==7.10.1
jupyter==1.0.0
jupyter-client==5.3.4
jupyter-console==6.0.0
jupyter-core==4.6.1
jupyterlab==1.2.4
jupyterlab-server==1.0.6
vega-datasets==0.7.0

In Jupyter lab open a new notebook and try:

from vega_datasets import data
import altair as alt

alt.data_transformers.enable('json')
alt.renderers.enable('default')

source = data.cars()

alt.Chart(source).mark_circle(size=60).encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
    tooltip=['Name', 'Origin', 'Horsepower', 'Miles_per_Gallon']
).interactive()

This leads to an empty chart with the following error in the JavaScript console:

vega@5?noext:1 GET http://localhost:8888/altair-data-4d45d69bbd706eda330e96e79ad4bf46.json 404 (Not Found)
vega@5?noext:1 WARN Infinite extent for field "Horsepower": [Infinity, -Infinity]
vega@5?noext:1 WARN Infinite extent for field "Miles_per_Gallon": [Infinity, -Infinity]

And in the Jupyter logs the error reads:

[W 16:56:10.144 LabApp] 404 GET /altair-data-4d45d69bbd706eda330e96e79ad4bf46.json (::1) 1.60ms referer=http://localhost:8888/lab

If I revert to altair==3.3.0 this error disappears and the data gets correctly loaded from disk and rendered. In that case the data URL (read from from the browser dev tools) becomes http://localhost:8888/files/altair-data-4d45d69bbd706eda330e96e79ad4bf46.json?_xsrf=<blah> (where I replaced the long string at the end with <blah).

[jakevdp]

With #1872, in the next version of Altair (4.1 or newer) you’ll be able to run

alt.data_transformers.enable('json', urlpath='files')

to make the transformer work correctly in JupyterLab with the default renderer. Without a way for the backend to detect what frontend you’re using (an idea the Jupyter team has long resisted) I don’t think there’s any way to make this more automatic.

[jakevdp]

The issue is that localhost filepaths are unpredictable between different flavors of Jupyter notebook.

I’d suggest avoiding the JSON/CSV transformer in favor of the data_server transformer, which should work regardless of what Jupyter frontend you’re on.