[bug] Color scheme is not customizable when using a `diverging` scale

[hydrosquall]

Bug Description

When using a diverging color scale as described in the docs, the color scheme is stuck at default blueorange

Below, I unsuccessfully tried to change the color scale to redblue (editor)

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "data": {
    "values": [
      {"x": 3, "y": 30, "value": -10},
      {"x": 4, "y": 40, "value": 0},
      {"x": 5, "y": 50, "value": 10}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "x", "type": "nominal"},
    "color": {
      "field": "value",
      "type": "quantitative",
      "scale": {"range": "diverging", "scheme": "redblue", "domainMid": 5}
    }
  }
}

Workaround: I get the intended behavior by removing "range": "diverging", and just set the custom "domainMid": 5.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "data": {
    "values": [
      {"x": 3, "y": 30, "value": -10},
      {"x": 4, "y": 40, "value": 0},
      {"x": 5, "y": 50, "value": 10}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "x", "type": "nominal"},
    "color": {
      "field": "value",
      "type": "quantitative",
      "scale": {
        "domainMid": 5,
        "scheme": "redblue"
      }
    }
  }
}

Potential next steps

1. Should range: diverging be updated to allow setting a named color scheme for the range?
2. If diverging is meant to always use this palette, does it make sense to update the docs for scale with an example of how to get a diverging color palette by choosing a diverging scheme from the collection, and setting a custom domainMid?

[domoritz]

I think it should work. We have alternative color ranges that one should be able to switch to.

Do we generate the correct Vega or is there an obvious mistake in the generated Vega?

[hydrosquall]

Do we generate the correct Vega or is there an obvious mistake in the generated Vega?

I think the generated scale is missing the redblue the string. If I make the scale linear, the generated vega 's color scale looks like this

    {
      "name": "color",
      "type": "linear",
      "domain": {"data": "data_0", "field": "value"},
      "range": {"scheme": "redblue"},
      "domainMid": 5,
      "interpolate": "hcl",
      "zero": false
    }

In contrast when I set the range to diverging, the scheme isn't passed through. It's not clear how the string diverging would merg with the object { "scheme": "some-named-scheme" }

   {
      "name": "color",
      "type": "linear",
      "domain": {"data": "data_0", "field": "value"},
      "range": "diverging",
      "domainMid": 5,
      "interpolate": "hcl",
      "zero": false
    }

[domoritz]

That's a bug then I'm pretty sure. Could you look into a pull request?