Improve Vega Lite to support validation

[ahuang11]

Migrate Vega validation from Lumen https://github.com/holoviz/lumen/blob/main/lumen/ai/views.py#L245-L262 into Panel.

Also handles archaic error:
Closes #8201

[MarcSkovMadsen]

What about airgapped usage of panel? Are we not trying to support that?

[ahuang11]

_download_vega_lite_schema is optional--only for validation.

            try:
                schema = self._download_vega_lite_schema(schema)
            except Exception as e:
                self.param.warning(f"Skipping validation because could not load Vega schema at {schema}: {e}")
                return

And users have full flexibility on using a local schema, meaning it can do something like: requests.get("localhost:8000/schema")?

However, potentially I can do open("schema").read()

[ahuang11]

Nevermind, open("schema").read() isn't feasible because I don't think the JS can access it, but if user exposes it as an endpoint, it's likely doable.

import json
import requests

with open("schema.json", "w") as f:
    json_file = requests.get("https://vega.github.io/schema/vega-lite/v5.json").json()
    json.dump(json_file, f)

import panel as pn
pn.extension('vega')

vegalite = {
  "$schema": "schema.json",
  "data": {"url": "https://raw.githubusercontent.com/vega/vega/master/docs/data/barley.json"},
  "mark": "bar",
  "encoding": {
    "x": {"aggregate": "sum", "field": "yield", "type": "quantitative"},
    "y": {"field": "variety", "type": "nominal"},
    "color": {"field": "site", "type": "nominal"}
  }
}
vgl_pane = pn.pane.Vega(vegalite, height=240)
vgl_pane.show()

[philippjfr]

At minimum this needs a timeout.

[Coderambling]

Nevermind, open("schema").read() isn't feasible because I don't think the JS can access it, but if user exposes it as an endpoint, it's likely doable.

Should a remark / example regarding air-gapped use be added to the Vega doc after this update?

Looks like that info is currently not part of the doc.

https://panel.holoviz.org/reference/panes/Vega.html

[Coderambling]

Related regarding air-gapped use: #7429

[codecov]

Codecov Report

❌ Patch coverage is 93.24324% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.24%. Comparing base (b749842) to head (5e884b5).

Files with missing lines	Patch %	Lines
panel/pane/vega.py	92.06%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (b749842) and HEAD (5e884b5). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (b749842)	HEAD (5e884b5)
	12	7

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #8215       +/-   ##
===========================================
- Coverage   86.01%   70.24%   -15.78%     
===========================================
  Files         348      347        -1     
  Lines       54378    54451       +73     
===========================================
- Hits        46775    38249     -8526     
- Misses       7603    16202     +8599     

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