refactoring: remove vega and vegalite wrappers

[mattijn]

Continuation of the discussion that started from this comment and onwards.

Vega 5 specifications can currently be rendered using Altair, with support of interactivity:

import altair.vega.v5 as alt
import json
vg_spec = r"""
{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "A basic bar chart example, with value labels shown upon mouse hover.",
  "width": 400,
  "height": 200,
  "padding": 5,
  "data": [
    {
      "name": "table",
      "values": [
        {"category": "A", "amount": 28},
        {"category": "B", "amount": 55},
        {"category": "C", "amount": 43},
        {"category": "D", "amount": 91},
        {"category": "E", "amount": 81},
        {"category": "F", "amount": 53},
        {"category": "G", "amount": 19},
        {"category": "H", "amount": 87}
      ]
    }
  ],
  "scales": [
    {
      "name": "xscale",
      "type": "band",
      "domain": {"data": "table", "field": "category"},
      "range": "width",
      "padding": 0.05,
      "round": true
    },
    {
      "name": "yscale",
      "domain": {"data": "table", "field": "amount"},
      "nice": true,
      "range": "height"
    }
  ],
  "axes": [
    {"orient": "bottom", "scale": "xscale"},
    {"orient": "left", "scale": "yscale"}
  ],
  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"scale": "xscale", "field": "category"},
          "width": {"scale": "xscale", "band": 1},
          "y": {"scale": "yscale", "field": "amount"},
          "y2": {"scale": "yscale", "value": 0}
        },
        "update": {"fill": {"value": "steelblue"}},
        "hover": {"fill": {"value": "red"}}
      }
    }
  ]
}
"""
alt.vega(json.loads(vg_spec))

Now, one can also use vl-convert (pip/conda install vl-convert-python) to render these as png or svg in a notebook:

import vl_convert as vlc
from IPython.display import Image
from IPython.display import SVG, display

#vg_png = vlc.vega_to_png(vg_spec)  # Image(vg_png)
vg_svg = vlc.vega_to_svg(vg_spec)
display(SVG(vg_svg))

But as far as I can tell it does not support the interactivity (btw just like a svg-export from the Vega Editor with above example).

Introducing errors in the spec, propagate JavaScript warnings back to the notebook, in both cases:
Changing eg: "from": {"data": "table"}, to "from": {"data": "FOO"},
Results with Altair variant:

Javascript Error: Undefined data set name: "FOO"

And with vl-convert variant:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[19], line 6
      3 from IPython.display import SVG, display
      5 #vg_png = vlc.vega_to_png(vg_spec)  # Image(vg_png)
----> 6 vg_svg = vlc.vega_to_svg(vg_spec)
      7 display(SVG(vg_svg))

ValueError: Vega to SVG conversion failed:
Error: Undefined data set name: "FOO"
    at b (https://cdn.skypack.dev/-/vega-util@v1.17.0-uRskU0IBL2vWCP4Va8OC/dist=es2020,mode=imports,min/optimized/vega-util.js:1:324)
    at ue.getData (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:45069)
    at At (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:21597)
    at Sa (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:21499)
    at Bt (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:24270)
    at https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:39099
    at Array.forEach (<anonymous>)
    at hn (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:39088)
    at al (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:39762)
    at Module.sl (https://cdn.skypack.dev/-/vega-parser@v6.1.4-PbzshukasZitnbnp29GQ/dist=es2020,mode=imports,min/optimized/vega-parser.js:1:47716)

Since it is not the core of Altair, we can decide for maintenance purposes to remove the Vega (currently v5) wrappers and leave this to vl-convert if people like to make use of this within a notebook environment.

Altair had until this commit support in the main version on Github support for the following Vega-Lite versions:

Altair Version	Vega-Lite Version	Vega Version
4.3.0.dev0	v3.4.0, v4.17.0, v5.2.0	v5.21.0

Three versions of Vega-Lite seems too many, and we are still discussing if we can reduce it to only a single version of Vega-Lite. But the v3 version is obsolete anyway. These packages/tests can safely be removed from main.

---

comment by @binste:

For completeness: JupyterLab has builtin support for Vega files which can be rendered with interactivity:

JupyterLab docs on this

You can also plot Vega specs in a notebook without converting to svg or png first so you still have the interactivity:

import json
from IPython.display import display

vg_spec = r"""
{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "A basic bar chart example, with value labels shown upon mouse hover.",
  "width": 400,
  "height": 200,
  "padding": 5,
  "data": [
    {
      "name": "table",
      "values": [
        {"category": "A", "amount": 28},
        {"category": "B", "amount": 55},
        {"category": "C", "amount": 43},
        {"category": "D", "amount": 91},
        {"category": "E", "amount": 81},
        {"category": "F", "amount": 53},
        {"category": "G", "amount": 19},
        {"category": "H", "amount": 87}
      ]
    }
  ],
  "scales": [
    {
      "name": "xscale",
      "type": "band",
      "domain": {"data": "table", "field": "category"},
      "range": "width",
      "padding": 0.05,
      "round": true
    },
    {
      "name": "yscale",
      "domain": {"data": "table", "field": "amount"},
      "nice": true,
      "range": "height"
    }
  ],
  "axes": [
    {"orient": "bottom", "scale": "xscale"},
    {"orient": "left", "scale": "yscale"}
  ],
  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"scale": "xscale", "field": "category"},
          "width": {"scale": "xscale", "band": 1},
          "y": {"scale": "yscale", "field": "amount"},
          "y2": {"scale": "yscale", "value": 0}
        },
        "update": {"fill": {"value": "steelblue"}},
        "hover": {"fill": {"value": "red"}}
      }
    }
  ]
}
"""

display({
    "application/vnd.vega.v5+json": json.loads(vg_spec)
}, raw=True)

The above code with display also works in VS Code and presumably a few other notebook frontends.

[mattijn]

Thanks for the comment @binste, given this alternative it is fine by me to refactor the Vega v5 wrappers out of Altair.

Regarding the Vega-Lite wrappers, another option would be to prepare a 4.3 version of Altair with some of the new features excluding the changes required for vega-lite v5 and from there refactor Altair v5 with just a single Vega-Lite v5 wrapper?

[joelostblom]

I am +1 on removing the Vega 5 code and it seems like Jake also had this in mind based on the comment that @binste linked.

Regarding a 4.3 version, I think there are some nice things with this, but it would involve a lot of rearranging or cherrypicking of commits to create the 4.3 release. I would rather spend that time on working toward a 5.0 release, and recommend people to update.

Based on the discussion in #2785 I remain in favor of only supporting Vega-Lite 5 in Altair 5. I agree with the idea that pinning of the Vega-Lite version is best done via pinning the Altair version, rather than using the newest version of Altair with specific Vega-Lite versions. It does also seem that this could add significant maintenance cost in terms of figuring out which new Altair features are safe to backport to the older Vega-Lite version and which should only be applied to the latest Vega-Lite version. Given that the upgrade from Altair 4 for 5 has been quite the undertaking, I'm can't help to lean towards the solution that is easier to maintain and upgrade in the future.

[mattijn]

Fine. At least its good documented why we are doing this. Then we agree to refactor vega v5, vega-lite v3 and vega-lite v4 out of Altair.

[joelostblom]

@jakevdp Just a ping here in case you are able to share your thoughts on this issue since it is a bigger decision. But I understand if you are not able to and then we can go ahead with the current plan for the time being.

[ChristopherDavisUCI]

Is Vega-Lite 5.2 what we're aiming for in the next release? On the Altair side, I don't think there will be any issue using a later version of Vega-Lite.

I'm asking here just as a possible reminder that this decision exists!

[mattijn]

Altair itself will be fine, but you won’t be able to pass the tests as defined in GitHub Action, since altair_viewer is still VL v5.2. If I recall correctly.

[joelostblom]

I had a brief look on the release notes for 5.3, 5.4, 5.5, and 5.6 and I don't see anything super important, so I am ok sticking with 5.2 and ironing out minor things in the meanwhile. But maybe open an issue to track it @ChristopherDavisUCI, in case it is something relatively straightforward we can do later, just before the release (candidate).

[ChristopherDavisUCI]

Thanks! @joelostblom I opened a brief issue here: #2833