How to use Vega/Vega-Lite specs in Altair, e.g. to create a circle packing plot?

Vega itself provides circle packing via the pack transform. You can find an example plot here.

I cannot find the pack transformation in the altair documentation. Is it somehow otherwise exposed or can I easily add it? I suppose, I can always modify the JSON Vega spec but that might be a bit brittle.

Altair is built on top of Vega-Lite, not directly on Vega. Therefore, the available transforms in Altair are those specified in Vega-Lite. Having that said, you could run Vega specs directly in Altair like this:

from altair.vega import Vega

Vega({
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "An example of a circle packing layout for hierarchical data.",
  "width": 600,
  "height": 600,
  "padding": 5,
  "autosize": "none",

  "data": [
    {
      "name": "tree",
      "url": "https://raw.githubusercontent.com/vega/vega/main/docs/data/flare.json",
      "transform": [
        {
          "type": "stratify",
          "key": "id",
          "parentKey": "parent"
        },
        {
          "type": "pack",
          "field": "size",
          "sort": {"field": "value"},
          "size": [{"signal": "width"}, {"signal": "height"}]
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "color",
      "type": "ordinal",
      "domain": {"data": "tree", "field": "depth"},
      "range": {"scheme": "category20"}
    }
  ],

  "marks": [
    {
      "type": "symbol",
      "from": {"data": "tree"},
      "encode": {
        "enter": {
          "shape": {"value": "circle"},
          "fill": {"scale": "color", "field": "depth"},
          "tooltip": {"signal": "datum.name + (datum.size ? ', ' + datum.size + ' bytes' : '')"}
        },
        "update": {
          "x": {"field": "x"},
          "y": {"field": "y"},
          "size": {"signal": "4 * datum.r * datum.r"},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 0.5}
        },
        "hover": {
          "stroke": {"value": "red"},
          "strokeWidth": {"value": 2}
        }
      }
    }
  ]
})

Note that the Vega wrapper in Altair might be deprecated in a future version of the package since it is now possible to use IPython to display Vega and Vega-Lite specs:

import json
from IPython.display import display

vg_spec = r"""{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "An example of a circle packing layout for hierarchical data.",
  "width": 600,
  "height": 600,
  "padding": 5,
  "autosize": "none",

  "data": [
    {
      "name": "tree",
      "url": "https://raw.githubusercontent.com/vega/vega/main/docs/data/flare.json",
      "transform": [
        {
          "type": "stratify",
          "key": "id",
          "parentKey": "parent"
        },
        {
          "type": "pack",
          "field": "size",
          "sort": {"field": "value"},
          "size": [{"signal": "width"}, {"signal": "height"}]
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "color",
      "type": "ordinal",
      "domain": {"data": "tree", "field": "depth"},
      "range": {"scheme": "category20"}
    }
  ],

  "marks": [
    {
      "type": "symbol",
      "from": {"data": "tree"},
      "encode": {
        "enter": {
          "shape": {"value": "circle"},
          "fill": {"scale": "color", "field": "depth"},
          "tooltip": {"signal": "datum.name + (datum.size ? ', ' + datum.size + ' bytes' : '')"}
        },
        "update": {
          "x": {"field": "x"},
          "y": {"field": "y"},
          "size": {"signal": "4 * datum.r * datum.r"},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 0.5}
        },
        "hover": {
          "stroke": {"value": "red"},
          "strokeWidth": {"value": 2}
        }
      }
    }
  ]
}
"""

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