{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "A contour plot example, overlaying a density estimate on scatter plot points.",
  "width": 500,
  "height": 400,
  "padding": 5,
  "autosize": "pad",

  "signals": [
    {
      "name": "bandwidth", "value": -1,
      "bind": {"input": "range", "min": -1, "max": 100, "step": 1}
    },
    {
      "name": "resolve", "value": "shared",
      "bind": {"input": "select", "options": ["independent", "shared"]}
    },
    {
      "name": "counts", "value": true,
      "bind": {"input": "checkbox"}
    }
  ],

  "data": [
    {
      "name": "source",
      "url": "data/cars.json",
      "transform": [
        {
          "type": "filter",
          "expr": "datum.Horsepower != null && datum.Miles_per_Gallon != null"
        }
      ]
    },
    {
      "name": "density",
      "source": "source",
      "transform": [
        {
          "type": "kde2d",
          "groupby": ["Origin"],
          "size": [{"signal": "width"}, {"signal": "height"}],
          "x": {"expr": "scale('x', datum.Horsepower)"},
          "y": {"expr": "scale('y', datum.Miles_per_Gallon)"},
          "bandwidth": {"signal": "[bandwidth, bandwidth]"},
          "counts": {"signal": "counts"}
        }
      ]
    },
    {
      "name": "contours",
      "source": "density",
      "transform": [
        {
          "type": "isocontour",
          "field": "grid",
          "resolve": {"signal": "resolve"},
          "levels": 3
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "linear",
      "round": true,
      "nice": true,
      "zero": true,
      "domain": {"data": "source", "field": "Horsepower"},
      "range": "width"
    },
    {
      "name": "y",
      "type": "linear",
      "round": true,
      "nice": true,
      "zero": true,
      "domain": {"data": "source", "field": "Miles_per_Gallon"},
      "range": "height"
    },
    {
      "name": "color",
      "type": "ordinal",
      "domain": {
        "data": "source", "field": "Origin",
        "sort": {"order": "descending"}
      },
      "range": "category"
    }
  ],

  "axes": [
    {
      "scale": "x",
      "grid": true,
      "domain": false,
      "orient": "bottom",
      "tickCount": 5,
      "title": "Horsepower"
    },
    {
      "scale": "y",
      "grid": true,
      "domain": false,
      "orient": "left",
      "titlePadding": 5,
      "title": "Miles_per_Gallon"
    }
  ],

  "legends": [
    {"stroke": "color", "symbolType": "stroke"}
  ],

  "marks": [
    {
      "name": "marks",
      "type": "symbol",
      "from": {"data": "source"},
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "Horsepower"},
          "y": {"scale": "y", "field": "Miles_per_Gallon"},
          "size": {"value": 4},
          "fill": {"value": "#ccc"}
        }
      }
    },
    {
      "type": "image",
      "from": {"data": "density"},
      "encode": {
        "update": {
          "x": {"value": 0},
          "y": {"value": 0},
          "width": {"signal": "width"},
          "height": {"signal": "height"},
          "aspect": {"value": false}
        }
      },
      "transform": [
        {
          "type": "heatmap",
          "field": "datum.grid",
          "resolve": {"signal": "resolve"},
          "color": {"expr": "scale('color', datum.datum.Origin)"}
        }
      ]
    },
    {
      "type": "path",
      "clip": true,
      "from": {"data": "contours"},
      "encode": {
        "enter": {
          "strokeWidth": {"value": 1},
          "strokeOpacity": {"value": 1},
          "stroke": {"scale": "color", "field": "Origin"}
        }
      },
      "transform": [
        { "type": "geopath", "field": "datum.contour" }
      ]
    }
  ]
}
