Javascript Vegalite：包含多个字段值的图例标签表达式

我的代码如下；

{
      "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
      "width": 800,
      "height": 200,
      "data": {
        "values": [
          {
            "pre_120": 0,
            "pre_90": 0,
            "pre_60": 0,
            "post_60": 100,
            "post_90": 150,
            "post_120": 200,
            "type": "Mango",
            "count": "twenty"
          },
          {
            "pre_120": 0,
            "pre_90": 0,
            "pre_60": 0,
            "post_60": 90,
            "post_90": 140,
            "post_120": 190,
            "type": "Apple",
            "count": "ten"
          }
        ]
      },
      "transform": [
        {"fold": ["pre_120", "pre_90", "pre_60","0", "post_60", "post_90", "post_120"]}
      ],
      "layer": [
        {
          "mark": "line",
          "encoding": {
            "x": {
              "field": "key",
              "sort": [
                "pre_120",
                "pre_90",
                "pre_60",
                "0",
                "post_60",
                "post_90",
                "post_120"
              ]
            },
            "y": {"field": "value", "type": "quantitative"},
            "color": {"field": "type", "type": "nominal"}
          }
        },
  {
    "mark": { 
      "type": "rule", 
      "color": "maroon", 
      "size": 3 ,
      "strokeDash": [6, 4]},
    "encoding": {
      "x": { "datum": "0" }
    }
  }
  ]
  }

下面是织女星编辑器的链接.

https://vega.github.io/editor/#/gist/947b2a99801bc6a08d468cdb9acbeb50/legend.json个

My aim is to show the legend to include value of 2 fields i.e. type and count fields with a hyphen. Example :
In the image below, I wanted to show the legends as
'Apple - twenty'
and
'Mango - ten'
How could I do the same?