Vega: what happens when the domain of a diverging color scale doesn't include its domainMid value

Add-negative-one.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "background": "white",
  "padding": 5,
  "style": "cell",
  "data": [
    {
      "name": "example-data",
      "values": [
        {"y": 5, "value": -1},
        {"y": 5, "value": 1},
        {"y": 5, "value": 2},
        {"y": 5, "value": 4},
        {"y": 5, "value": 8},
        {"y": 5, "value": 16},
        {"y": 5, "value": 32}
      ]
    }
  ],
  "scales": [
    {
      "name": "color",
      "type": "linear",
      "domain": {"data": "example-data", "field": "value"},
      "range": {"scheme": "redblue"},
      "domainMid": 0,
      "interpolate": "hcl",
      "zero": false
    },
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "example-data", "field": "value"},
      "range": {"step": {"signal": "x_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    },
    {
      "name": "y",
      "type": "band",
      "domain": {"data": "example-data", "field": "y"},
      "range": {"step": {"signal": "y_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    }
  ],
  "signals": [
    {"name": "x_step", "value": 20},
    {"name": "width", "update": "bandspace(domain('x').length, 0, 0) * x_step"},
    {"name": "y_step", "value": 20},
    {"name": "height", "update": "bandspace(domain('y').length, 0, 0) * y_step"}
  ],
  "marks": [
    {
      "name": "marks",
      "type": "rect",
      "style": ["rect"],
      "from": {"data": "example-data"},
      "encode": {
        "update": {
          "fill": {"scale": "color", "field": "value"},
          "x": {"scale": "x", "field": "value"},
          "width": {"scale": "x", "band": 1},
          "height": {"scale": "y", "band": 1}
        }
      }
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "value",
      "labelAlign": "right",
      "labelAngle": 270,
      "labelBaseline": "middle",
      "labelOverlap": true,
      "zindex": 1
    }
  ],
  "legends": [
    {
      "fill": "color",
      "gradientLength": {"signal": "clamp(height, 64, 200)"},
      "symbolType": "square",
      "title": "value"
    }
  ]
}

Domain-union-with-zero.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "background": "white",
  "padding": 5,
  "style": "cell",
  "data": [
    {
      "name": "example-data",
      "values": [
        {"y": 5, "value": 1},
        {"y": 5, "value": 2},
        {"y": 5, "value": 4},
        {"y": 5, "value": 8},
        {"y": 5, "value": 16},
        {"y": 5, "value": 32}
      ]
    }
  ],
  "scales": [
    {
      "name": "color",
      "type": "linear",
      "domain": {
        "fields": [
          {
            "data": "example-data", 
            "field": "value"
          }, 
          [0]
        ]
      },
      "range": {"scheme": "redblue"},
      "domainMid": 0,
      "interpolate": "hcl",
      "zero": false
    },
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "example-data", "field": "value"},
      "range": {"step": {"signal": "x_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    },
    {
      "name": "y",
      "type": "band",
      "domain": {"data": "example-data", "field": "y"},
      "range": {"step": {"signal": "y_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    }
  ],
  "signals": [
    {"name": "x_step", "value": 20},
    {"name": "width", "update": "bandspace(domain('x').length, 0, 0) * x_step"},
    {"name": "y_step", "value": 20},
    {"name": "height", "update": "bandspace(domain('y').length, 0, 0) * y_step"}
  ],
  "marks": [
    {
      "name": "marks",
      "type": "rect",
      "style": ["rect"],
      "from": {"data": "example-data"},
      "encode": {
        "update": {
          "fill": {"scale": "color", "field": "value"},
          "x": {"scale": "x", "field": "value"},
          "width": {"scale": "x", "band": 1},
          "height": {"scale": "y", "band": 1}
        }
      }
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "value",
      "labelAlign": "right",
      "labelAngle": 270,
      "labelBaseline": "middle",
      "labelOverlap": true,
      "zindex": 1
    }
  ],
  "legends": [
    {
      "fill": "color",
      "gradientLength": {"signal": "clamp(height, 64, 200)"},
      "symbolType": "square",
      "title": "value"
    }
  ]
}

Reproduction.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "background": "white",
  "padding": 5,
  "style": "cell",
  "data": [
    {
      "name": "example-data",
      "values": [
        {"y": 5, "value": 1},
        {"y": 5, "value": 2},
        {"y": 5, "value": 4},
        {"y": 5, "value": 8},
        {"y": 5, "value": 16},
        {"y": 5, "value": 32}
      ]
    }
  ],
  "scales": [
    {
      "name": "color",
      "type": "linear",
      "domain": {"data": "example-data", "field": "value"},
      "range": {"scheme": "redblue"},
      "domainMid": 0,
      "interpolate": "hcl",
      "zero": false
    },
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "example-data", "field": "value"},
      "range": {"step": {"signal": "x_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    },
    {
      "name": "y",
      "type": "band",
      "domain": {"data": "example-data", "field": "y"},
      "range": {"step": {"signal": "y_step"}},
      "paddingInner": 0,
      "paddingOuter": 0
    }
  ],
  "signals": [
    {"name": "x_step", "value": 20},
    {"name": "width", "update": "bandspace(domain('x').length, 0, 0) * x_step"},
    {"name": "y_step", "value": 20},
    {"name": "height", "update": "bandspace(domain('y').length, 0, 0) * y_step"}
  ],
  "marks": [
    {
      "name": "marks",
      "type": "rect",
      "style": ["rect"],
      "from": {"data": "example-data"},
      "encode": {
        "update": {
          "fill": {"scale": "color", "field": "value"},
          "x": {"scale": "x", "field": "value"},
          "width": {"scale": "x", "band": 1},
          "height": {"scale": "y", "band": 1}
        }
      }
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "value",
      "labelAlign": "right",
      "labelAngle": 270,
      "labelBaseline": "middle",
      "labelOverlap": true,
      "zindex": 1
    }
  ],
  "legends": [
    {
      "fill": "color",
      "gradientLength": {"signal": "clamp(height, 64, 200)"},
      "symbolType": "square",
      "title": "value"
    }
  ]
}