Altair plots cannot be embedded into html when panel is run from a script

[joelostblom]

Embeding Altair plots to an html file from a notebok works, and the saved file from the code below has embedded the plot for the different selections in the dropdown as expected. However, running it from a script does not work. I am thinking it has something to do with the vega extension needing the notebook environment to create an Altair plot, but might be wrong. If this is the case, maybe it would be useful to have a warning when trying to run this from a script, since embedding other components such as text etc works as expected.

import altair as alt
from vega_datasets import data
import panel as pn
from panel.interact import interact


pn.extension('vega')

cars = data.cars()

def scatter_plot(x_column):
    chart = alt.Chart(cars).mark_point().encode(
        x=x_column,
        y='Displacement')
    return chart

interact(scatter_plot, x_column=cars.select_dtypes('number').columns).save('test3.html', embed=True)

ALL software version info

session_info.show()

-----
altair              4.1.0
pandas              1.2.4
panel               0.11.3
session_info        1.0.0
vega_datasets       0.9.0
-----
IPython             7.24.1
jupyter_client      6.1.12
jupyter_core        4.7.1
jupyterlab          2.3.1
notebook            6.4.0
-----
Python 3.8.10 | packaged by conda-forge | (default, May 11 2021, 07:01:05) [GCC 9.3.0]
Linux-5.11.0-7614-generic-x86_64-with-glibc2.10