Nested concatenated altair charts with `vconcat(hconcat(...), ...)` cannot be shown using mo.ui.altair_chart

[andycasey]

Describe the bug

I seem to be experiencing a bug with nested charts.

If I use the code to reproduce that I have given below, here is what the plot looks like to me:

If I re-arrange the nesting like in #5015, then it works fine. But when I rearrange to have an outer vconcat and an inner hconcat, all plots disappear.

Will you submit a PR?

• Yes

Environment

{
  "marimo": "0.15.3",
  "editable": false,
  "OS": "Darwin",
  "OS Version": "23.5.0",
  "Processor": "i386",
  "Python Version": "3.12.5",
  "Binaries": {
    "Browser": "140.0.7339.133",
    "Node": "v22.18.0"
  },
  "Dependencies": {
    "click": "8.2.1",
    "docutils": "0.21.2",
    "itsdangerous": "2.2.0",
    "jedi": "0.19.2",
    "markdown": "3.9",
    "narwhals": "2.4.0",
    "packaging": "25.0",
    "psutil": "7.0.0",
    "pygments": "2.19.2",
    "pymdown-extensions": "10.16.1",
    "pyyaml": "6.0.2",
    "starlette": "0.47.3",
    "tomlkit": "0.13.3",
    "typing-extensions": "4.14.1",
    "uvicorn": "0.35.0",
    "websockets": "15.0.1"
  },
  "Optional Dependencies": {
    "altair": "5.5.0",
    "pandas": "2.3.1",
    "pyarrow": "21.0.0",
    "loro": "1.6.0"
  },
  "Experimental Flags": {}
}

Code to reproduce

from vega_datasets import data
import marimo as mo
import altair as alt

cars = data.cars()

_chart = (
    alt.Chart(cars)
    .mark_point()
    .encode(
        x="Horsepower",
        y="Miles_per_Gallon",
        color="Origin",
    )
    .properties(height=100)
)

stacked_chart = alt.vconcat(
    alt.hconcat(
        _chart,
        _chart,
    ),
    _chart
)


mo.ui.altair_chart(stacked_chart)

[mscolnick]

This is now fixed by #6970