{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 700,
  "height": 400,
  "padding": 5,
  "background": "#edf1f7",

  "config": {
    "axisBand": {
      "bandPosition": 0,
      "labelPadding": 5,
      "tickOffset": 0
    }
  },

  "signals": [
    {
      "name": "dragging",
      "value": false,
      "on": [
        {"events": "@handle:mousedown", "update": "true"},
        {"events": "window:mouseup", "update": "false"}
      ]
    },
    {
      "name": "handleYear",
      "value": 2010,
      "on": [{
        "events": "[@handle:mousedown, window:mouseup] > window:mousemove!",
        "update": "invert('x', clamp(x(), 0, width))"
      }]
    },
    {
      "name": "currentYear",
      "update": "clamp(handleYear, 1980, 2010)"
    },
    {
      "name": "tipYear",
      "on": [{
        "events": "mousemove",
        "update": "dragging ? tipYear : invert('x', x())"
      }]
    },
    {
      "name": "tipValue",
      "on": [{
        "events": "mousemove",
        "update": "dragging ? tipValue : invert('y', y())"
      }]
    },
    {
      "name": "cursor", "value": "default",
      "on": [{
        "events": {"signal": "dragging"},
        "update": "dragging ? 'pointer' : 'default'"
      }]
    }
  ],

  "data": [
    {
      "name": "budgets",
      "url": "data/budgets.json",
      "transform": [
        { "type": "formula", "as": "abs", "expr": "abs(datum.value)" },
        { "type": "formula", "as": "type", "expr": "datum.value < 0 ? 'deficit' : 'surplus'" }
      ]
    },
    {
      "name": "budgets-current",
      "source": "budgets",
      "transform": [
        { "type": "filter", "expr": "datum.budgetYear <= currentYear" }
      ]
    },
    {
      "name": "budgets-actual",
      "source": "budgets",
      "transform": [
        { "type": "filter", "expr": "datum.budgetYear <= currentYear && datum.forecastYear == datum.budgetYear - 1" }
      ]
    },
    {
      "name": "tooltip",
      "source": "budgets",
      "transform": [
        {
          "type": "filter",
          "expr": "datum.budgetYear <= currentYear && datum.forecastYear == tipYear && abs(datum.value - tipValue) <= 0.1"
        },
        {
          "type": "aggregate",
          "fields": ["value", "value"],
          "ops": ["min", "argmin"],
          "as": ["min", "argmin"]
        },
        { "type": "formula", "as": "tooltipYear", "expr": "datum.argmin.budgetYear" }
      ]
    },
    {
      "name": "tooltip-forecast",
      "source": "budgets",
      "transform": [
        {
          "type": "lookup",
          "from": "tooltip", "key": "tooltipYear",
          "fields": ["budgetYear"], "as": ["tooltip"]
        },
        { "type": "filter", "expr": "datum.tooltip" }
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "budgets", "field": "forecastYear"},
      "range": "width"
    },
    {
      "name": "y",
      "type": "linear", "zero": true,
      "domain": {"data": "budgets", "field": "value"},
      "range": "height"
    }
  ],

  "axes": [
    {
      "orient": "bottom", "scale": "x",
      "grid": true, "domain": false,
      "values": [1982, 1986, 1990, 1994, 1998, 2002, 2006, 2010, 2014, 2018],
      "tickSize": 0,
      "gridColor": "white",
      "gridOpacity": 0.75,
      "encode": {
        "labels": {
          "update": {
            "x": {"scale": "x", "field": "value"}
          }
        }
      }
    },
    {
      "orient": "right", "scale": "y",
      "grid": true, "domain": false,
      "values": [0, -0.5, -1, -1.5],
      "tickSize": 0,
      "gridColor": "white",
      "gridOpacity": 0.75,
      "encode": {
        "labels": {
          "enter": {
            "text": {"signal": "format(datum.value, '$.1f') + ' trillion'"}
          }
        }
      }
    }
  ],

  "marks": [
    {
      "type": "group",
      "from": {
        "facet": {
          "name": "facet",
          "data": "budgets-current",
          "groupby": "budgetYear"
        }
      },

      "marks": [
        {
          "type": "line",
          "from": {"data": "facet"},
          "encode": {
            "update": {
              "x": {"scale": "x", "field": "forecastYear"},
              "y": {"scale": "y", "field": "value"},
              "stroke": {"value": "steelblue"},
              "strokeWidth": {"value": 1},
              "strokeOpacity": {"value": 0.25}
            }
          }
        }
      ]
    },
    {
      "type": "line",
      "from": {"data": "budgets-actual"},
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "forecastYear"},
          "y": {"scale": "y", "field": "value"},
          "stroke": {"value": "steelblue"},
          "strokeWidth": {"value": 3}
        }
      }
    },

    {
      "type": "line",
      "from": {"data": "tooltip-forecast"},
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "forecastYear"},
          "y": {"scale": "y", "field": "value"},
          "stroke": {"value": "black"},
          "strokeWidth": {"value": 1}
        }
      }
    },
    {
      "type": "symbol",
      "from": {"data": "tooltip"},
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "argmin.forecastYear"},
          "y": {"scale": "y", "field": "argmin.value"},
          "size": {"value": 50},
          "fill": {"value": "black"}
        }
      }
    },

    {
      "type": "rule",
      "encode": {
        "enter": {
          "stroke": {"value": "#000"},
          "strokeWidth": {"value": 1}
        },
        "update": {
          "x": {"value": 0},
          "x2": {"scale": "x", "signal": "currentYear"},
          "y": {"scale": "y", "value": 0}
        }
      }
    },
    {
      "name": "handle",
      "type": "symbol",
      "encode": {
        "enter": {
          "shape": {"value": "triangle-down"},
          "size": {"value": 400},
          "stroke": {"value": "#000"},
          "strokeWidth": {"value": 0.5}
        },
        "update": {
          "x": {"scale": "x", "signal": "currentYear"},
          "y": {"scale": "y", "value": 0, "offset": 1},
          "fill": {"signal": "dragging ? 'lemonchiffon' : '#fff'"}
        },
        "hover": {
          "fill": {"value": "lemonchiffon"},
          "cursor": {"value": "pointer"}
        }
      }
    },
    {
      "type": "text",
      "encode": {
        "enter": {
          "x": {"value": 0},
          "y": {"value": 25},
          "fontSize": {"value": 32},
          "fontWeight": {"value": "bold"},
          "fill": {"value": "steelblue"}
        },
        "update": {
          "text": {"signal": "currentYear"}
        }
      }
    },

    {
      "type": "group",
      "from": {"data": "tooltip"},
      "interactive": false,
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "argmin.forecastYear", "offset": -5},
          "y": {"scale": "y", "field": "argmin.value", "offset": 20},
          "width": {"value": 150},
          "height": {"value": 35},
          "fill": {"value": "#fff"},
          "fillOpacity": {"value": 0.85},
          "stroke": {"value": "#aaa"},
          "strokeWidth": {"value": 0.5}
        }
      },
      "marks": [
        {
          "type": "text",
          "interactive": false,
          "encode": {
            "update": {
              "x": {"value": 6},
              "y": {"value": 14},
              "text": {"signal": "'Forecast from early ' + parent.argmin.budgetYear"},
              "fill": {"value": "black"},
              "fontWeight": {"value": "bold"}
            }
          }
        },
        {
          "type": "text",
          "interactive": false,
          "encode": {
            "update": {
              "x": {"value": 6},
              "y": {"value": 29},
              "text": {"signal": "parent.argmin.forecastYear + ': ' + format(parent.argmin.abs, '$.3f') + ' trillion ' + parent.argmin.type"},
              "fill": {"value": "black"},
              "align": {"value": "left"}
            }
          }
        }
      ]
    }
  ]
}