Density histogram with a vertical rule and some other formatting

spec.json

/*
Density histogram with a vertical rule and some other formatting.
*/
{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "datasets": {
    "linedata": [
      {"b": 5},
      {"b": 20},
      {"b": 20},
      {"b": 20},
      {"b": 40},
      {"b": 25},
      {"b": 20},
      {"b": 15},
      {"b": 100}
    ],
    "ruledata": {"ownval": 25}
  },
  "layer": [
    {
      "data": {"name": "linedata"},
      "width": 600,
      "height": 500,
      "transform": [{"density": "b", "bandwidth": 15}],
      "mark": {"type": "area", "color": "#663399"},
      "encoding": {
        "x": {
          "field": "value",
          "type": "quantitative",
          "axis": {
            "title": "Average Hours",
            "ticks": 5,
            "labelFontSize": 15,
            "titleFontSize": 15,
            "tickMinStep": 10
          }
        },
        "y": {"field": "density", "type": "quantitative", "axis": false}
      }
    },
    {
      "data": {"name": "ruledata"},
      "mark": {
        "type": "rule",
        "strokeDash": [4, 4],
        "size": 3,
        "color": "black"
      },
      "encoding": {"x": {"field": "ownval", "type": "quantitative"}}
    }
  ]
}