Empty plot when enabling json data_transformer

Below is my code, I get an empty plot. I am using JupyterLab

import altair as alt
import pandas as pd
alt.data_transformers.enable("json")

size = 10
df = pd.DataFrame(
    {"key": np.random.choice(list("ABC"), size), "val": np.random.rand(size)}
)
alt.Chart(df).mark_point().encode(y="key:N")

My versions are: altair: 3.2.0 jupyterlab: 1.0.4

I tried looking through the page response when inspecting the network activity on Chrome, I am seeing a forbidden error.

<div class="error">
    <h1>403 : Forbidden</h1>
    <p>The error was:</p>
    <div class="traceback-wrapper">
    <pre class="traceback"> is not a file</pre>
    </div>
</div>

Below is the to_dict() from the altair object.

{'config': {'view': {'width': 400, 'height': 300}, 'mark': {'tooltip': None}},
 'data': {'url': 'altair-data-e47f5911f8d2367541736476ae610606.json',
  'format': {'type': 'json'}},
 'mark': 'point',
 'encoding': {'y': {'type': 'nominal', 'field': 'key'}},
 '$schema': 'https://vega.github.io/schema/vega-lite/v3.4.0.json'}

and the datafile exists in the local path

[jakevdp]

This looks like the same issue as reported in https://github.com/jupyterlab/jupyterlab/issues/7017, which has been fixed as of JupyterLab version 1.1.0. Based on this, I believe that updating Jupyterlab will fix your issue.

[rschwarz]

I also have the same behavior as the above two posts, using versions:

altair               4.2.0
jupyter-client       7.1.2
jupyter-core         4.9.2
jupyter-server       1.13.5
jupyterlab           3.2.9
jupyterlab-server    2.10.3

While passing a pandas DataFrame works, using any of the two transformers does not. Neither does manually saving to CSV and refering to that filename.

UPDATE: To expand on the experiment with manual CSV saving, here are a couple of URLs I tried, some of which worked. In this case, I did not enable any data_transformer and saved the file to foobar.csv next to the notebook files, which is also from where I started JupyterLab.

FAIL: "foobar.csv"     # but works with classic Jupyter Notebook      
FAIL: "files/foobar.csv"
OK:   "/files/foobar.csv"
OK:   "http://localhost:8888/files/foobar.csv"
FAIL: "http://localhost:8888/files/foobar.csv?_xsrf=2%7Ce1d7e552%7Cd48f09f31845ffbba0881670bae6d705%7C1645528121"
FAIL: "http://localhost:8888/lab/tree/34.fcoagg.csv"

Here, the latter two are the URLs which I get whith right-click on the CSV file and then either “Copy download link” or “Copy shareable link”.

To conclude, the URL that works is the “download link” with the query parameter removed.