如何在Vega中基于数据列设置标记的opacity？现有配置未生效

当然可以基于数据列控制opacity值，你要实现的「c=1时opacity=1，c=0时opacity=0」完全可行。当前代码的逻辑方向没问题，但可以优化写法，确保设置能正确生效。

解决方案 #

你已经通过facet按c字段分组，每个分组内的datum.c值是统一的（要么全0要么全1），因此可以直接将opacity绑定到c字段——因为c的值正好是0或1，完全匹配opacity的取值范围，写法更简洁：

修改symbol标记的opacity设置：

"opacity": {"field": "c"}

如果偏好保留条件判断的写法，也可以调整为：

"opacity": {"condition": {"test": "datum.c === 0", "value": 0}, "value": 1}

如果希望折线也跟随c值调整透明度，只需在line标记的enter或update块中添加同样的opacity设置即可。

修改后的完整代码 #

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "A basic line chart example.",
  "width": 500,
  "height": 200,
  "padding": 5,

  "signals": [
    {
      "name": "interpolate",
      "value": "linear",
      "bind": {
        "input": "select",
        "options": [
          "basis",
          "cardinal",
          "catmull-rom",
          "linear",
          "monotone",
          "natural",
          "step",
          "step-after",
          "step-before"
        ]
      }
    }
  ],

  "data": [
    {
      "name": "table",
      "values": [
        {"x": 0, "y": 28, "c":0}, {"x": 0, "y": 20, "c":1},
        {"x": 1, "y": 43, "c":0}, {"x": 1, "y": 35, "c":1},
        {"x": 2, "y": 81, "c":0}, {"x": 2, "y": 10, "c":1},
        {"x": 3, "y": 19, "c":0}, {"x": 3, "y": 15, "c":1},
        {"x": 4, "y": 52, "c":0}, {"x": 4, "y": 48, "c":1},
        {"x": 5, "y": 24, "c":0}, {"x": 5, "y": 28, "c":1},
        {"x": 6, "y": 87, "c":0}, {"x": 6, "y": 66, "c":1},
        {"x": 7, "y": 17, "c":0}, {"x": 7, "y": 27, "c":1},
        {"x": 8, "y": 68, "c":0}, {"x": 8, "y": 16, "c":1},
        {"x": 9, "y": 49, "c":0}, {"x": 9, "y": 25, "c":1}
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "point",
      "range": "width",
      "domain": {"data": "table", "field": "x"}
    },
    {
      "name": "y",
      "type": "linear",
      "range": "height",
      "nice": true,
      "zero": true,
      "domain": {"data": "table", "field": "y"}
    },
    {
      "name": "color",
      "type": "ordinal",
      "range": "category",
      "domain": {"data": "table", "field": "c"}
    }
  ],

  "axes": [
    {"orient": "bottom", "scale": "x"},
    {"orient": "left", "scale": "y"}
  ],

  "marks": [
    {
      "type": "group",
      "from": {
        "facet": {
          "name": "series",
          "data": "table",
          "groupby": "c"
        }
      },
      "marks": [
        {
          "type": "line",
          "from": {"data": "series"},
          "encode": {
            "enter": {
              "x": {"scale": "x", "field": "x"},
              "y": {"scale": "y", "field": "y"},
              "stroke": {"scale": "color", "field": "c"},
              "strokeWidth": {"value": 2},
              "opacity": {"field": "c"}
            },
            "update": {
              "interpolate": {"signal": "interpolate"},
              "strokeOpacity": {"value": 1}
            },
            "hover": {
              "strokeOpacity": {"value": 0.5}
            }
          }
        },
        {
          "type": "symbol",
          "from": {"data": "series"},
          "encode": {
            "enter": {
              "x": {"scale": "x", "field": "x"},
              "y": {"scale": "y", "field": "y"},
              "stroke": {"scale": "color", "field": "c"},
              "strokeWidth": {"value": 2},
              "fill": {"scale": "color", "field": "c"},
              "opacity": {"field": "c"}
            },
            "update": {
              "interpolate": {"signal": "interpolate"},
              "strokeOpacity": {"value": 1}
            },
            "hover": {
              "strokeOpacity": {"value": 0.5}
            }
          }
        }
      ]
    }
  ]
}

内容的提问来源于stack exchange，提问作者Osman Mamun