{
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  "padding": 5,
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      "name": "cross_column_domain_row_domain",
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          "type": "aggregate",
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      "name": "column_domain",
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          "type": "aggregate",
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          "fields": ["distinct_a"],
          "ops": ["max"],
          "as": ["distinct_a"]
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      "name": "row_domain",
      "source": "cross_column_domain_row_domain",
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          "type": "aggregate",
          "groupby": ["r"],
          "fields": ["distinct_b"],
          "ops": ["max"],
          "as": ["distinct_b"]
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    "padding": 20,
    "offset": {"rowTitle": 10, "columnTitle": 10},
    "columns": {"signal": "length(data('column_domain'))"},
    "bounds": "full",
    "align": "all"
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        "orient": "left",
        "style": "guide-title",
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      }
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      "name": "column-title",
      "type": "group",
      "role": "column-title",
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      "name": "row_header",
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      "sort": {"field": "datum[\"r\"]", "order": "ascending"},
      "title": {
        "text": {
          "signal": "isValid(parent[\"r\"]) ? parent[\"r\"] : \"\"+parent[\"r\"]"
        },
        "orient": "left",
        "style": "guide-label",
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      "name": "cell",
      "type": "group",
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            "signal": "bandspace(datum[\"distinct_a\"], 0, 0) * child_x_step"
          },
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            "signal": "bandspace(datum[\"distinct_b\"], 0, 0) * child_y_step"
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        }
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        {
          "name": "child_marks",
          "type": "rect",
          "style": ["rect"],
          "from": {"data": "facet"},
          "encode": {
            "update": {
              "fill": {"value": "#4c78a8"},
              "description": {
                "signal": "\"a: \" + (isValid(datum[\"a\"]) ? datum[\"a\"] : \"\"+datum[\"a\"]) + \"; b: \" + (isValid(datum[\"b\"]) ? datum[\"b\"] : \"\"+datum[\"b\"])"
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              "y": {"scale": "child_y", "field": "b"},
              "height": {"scale": "child_y", "band": 1}
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        }
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        {
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          "domain": {"data": "facet", "field": "a", "sort": true},
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          "paddingInner": 0,
          "paddingOuter": 0
        }
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        {
          "scale": "child_x",
          "orient": "bottom",
          "grid": false,
          "title": "a",
          "labelAlign": "right",
          "labelAngle": 270,
          "labelBaseline": "middle",
          "zindex": 1
        },
        {
          "scale": "child_y",
          "orient": "left",
          "grid": false,
          "title": "b",
          "zindex": 1
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      ]
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}