如何在Power BI的Deneb哑铃图中添加垂直平均线？

我正在Power BI的Deneb中制作哑铃图，想要添加一条垂直线代表所有数值的平均值。参考类似方案后，得到的却是多条平均值水平线，而非期望的单条垂直线（效果见下图）。我需要的是一条代表整体平均值0.73的垂直线。

复现数据集 #

当前使用的Vega-Lite代码 #

{  
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",  
  "description": "Dumbbell chart with labels",  
  "data":  {"name": "dataset"},    
  "transform": [  
    {"joinaggregate": [{  
      "op": "mean", "field": "Grand Mean", "as": "Grand Mean"  
    }]  
    }],  
  "encoding": {  
    "x": {  
      "field": "Value",  
      "type": "quantitative",  
      "title": "Factor Value"//,  
    },  
    "y": {  
      "field": "Categories",  
      "type": "nominal",  
      "title": null//,  
    }  
  },  
  "layer": [        
    {  
      "mark": "line",  
      "encoding": {  
        "detail": {  
          "field": "Categories",  
          "type": "nominal"  
        },  
        "color": {"value": "gray"}  
      }  
    },  
    {  
      "mark": {  
        "type": "point",  
        "filled": true  
      },  
      "encoding": {  
        "color": {  
          "field": "Factor",  
          "type": "ordinal",           
          "legend": {                
            "title": null,                
            "offset": 0          
          },  
          "scale": {  
            "domain": ["One", "Two"],  
            "range": ["#6ba5cd", "#8e946a"]  
          },  
          "title": "Legend"  
        },  
        "size": {"value": 300},  
        "opacity": {"value": 1}  
      }  
    },  
    {  
      "mark": {  
        "type": "text",  
        "align": "center",  
        "baseline": "bottom",  
        "dy": -15  
      },  
      "encoding": {  
        "detail": {  
          "field": "Categories",  
          "type": "nominal"  
        },  
        "text": {  
          "field": "Factor",  
          "type": "ordinal"  
        }  
      }  
    },  
    // adding in a red line:   
    {"mark": {"type": "rule", "color": "red"   },     
     "encoding": {  
      "x": {  
        "field": "Grand Mean", "type": "quantitative"}        
      }  
    }  
  ]  
}

问题原因及修复方案 #

问题根源 #

当前代码中，rule标记的图层继承了顶层的y编码（绑定到Categories字段），所以Vega-Lite会为每个分类生成一条水平线。同时，你用joinaggregate计算平均值的方式也冗余了——数据集里已经有全局平均值Grand Mean，不需要再重复计算。

修复步骤 #

1. 移除冗余的joinaggregate转换：因为数据集中已经包含全局平均值Grand Mean，直接使用即可。
2. 为垂直线图层重置y编码：在rule图层的编码中添加y配置，设置为{"type": "nominal", "domain": {"union": true}}，覆盖顶层的分类绑定，让垂直线跨越整个y轴范围，变成全局的垂直线。

修复后的完整代码 #

{  
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",  
  "description": "Dumbbell chart with labels",  
  "data":  {"name": "dataset"},  
  "encoding": {  
    "x": {  
      "field": "Value",  
      "type": "quantitative",  
      "title": "Factor Value"  
    },  
    "y": {  
      "field": "Categories",  
      "type": "nominal",  
      "title": null  
    }  
  },  
  "layer": [        
    {  
      "mark": "line",  
      "encoding": {  
        "detail": {  
          "field": "Categories",  
          "type": "nominal"  
        },  
        "color": {"value": "gray"}  
      }  
    },  
    {  
      "mark": {  
        "type": "point",  
        "filled": true  
      },  
      "encoding": {  
        "color": {  
          "field": "Factor",  
          "type": "ordinal",           
          "legend": {                
            "title": null,                
            "offset": 0          
          },  
          "scale": {  
            "domain": ["One", "Two"],  
            "range": ["#6ba5cd", "#8e946a"]  
          },  
          "title": "Legend"  
        },  
        "size": {"value": 300},  
        "opacity": {"value": 1}  
      }  
    },  
    {  
      "mark": {  
        "type": "text",  
        "align": "center",  
        "baseline": "bottom",  
        "dy": -15  
      },  
      "encoding": {  
        "detail": {  
          "field": "Categories",  
          "type": "nominal"  
        },  
        "text": {  
          "field": "Factor",  
          "type": "ordinal"  
        }  
      }  
    },  
    // 全局平均值垂直线  
    {  
      "mark": {"type": "rule", "color": "red"},     
      "encoding": {  
        "x": {"field": "Grand Mean", "type": "quantitative"},  
        "y": {"type": "nominal", "domain": {"union": true}}  
      }  
    }  
  ]  
}

额外说明 #

如果你的数据集里没有预先计算好的Grand Mean，可以用独立数据集计算全局平均值后绘制垂直线，示例如下：

// 在layers外添加单独的平均值计算
"data": [
  {"name": "dataset"},
  {"name": "mean-data", "source": "dataset", "transform": [{"aggregate": [{"op": "mean", "field": "Value", "as": "Grand Mean"}]}]}
],
// 垂直线图层改为使用mean-data
{
  "mark": {"type": "rule", "color": "red"},
  "data": {"name": "mean-data"},
  "encoding": {
    "x": {"field": "Grand Mean", "type": "quantitative"},
    "y": {"type": "nominal", "domain": {"union": true}}
  }
}

内容的提问来源于stack exchange，提问作者JR87