{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 800,
  "height": 600,
  "padding": 5,

  "data": [
    {
      "name": "gapminder",
      "url": "data/gapminder.json"
    },
    {
      "name": "clusters",
      "values": [
        {"id": 0, "name": "South Asia"},
        {"id": 1, "name": "Europe & Central Asia"},
        {"id": 2, "name": "Sub-Saharan Africa"},
        {"id": 3, "name": "America"},
        {"id": 4, "name": "East Asia & Pacific"},
        {"id": 5, "name": "Middle East & North Africa"}
      ]
    },
    {
      "name": "country_timeline",
      "source": "gapminder",
      "transform": [
        {"type": "filter", "expr": "timeline && datum.country == timeline.country"},
        {"type": "collect", "sort": {"field": "year"}}
      ]
    },
    {
      "name": "thisYear",
      "source": "gapminder",
      "transform": [
        {"type": "filter", "expr": "datum.year == currentYear"}
      ]
    },
    {
      "name": "prevYear",
      "source": "gapminder",
      "transform": [
        {"type": "filter", "expr": "datum.year == currentYear - stepYear"}
      ]
    },
    {
      "name": "nextYear",
      "source": "gapminder",
      "transform": [
        {"type": "filter", "expr": "datum.year == currentYear + stepYear"}
      ]
    },
    {
      "name": "countries",
      "source": "gapminder",
      "transform": [
        {"type": "aggregate", "groupby": ["country"]}
      ]
    },
    {
      "name": "interpolate",
      "source": "countries",
      "transform": [
        {
          "type": "lookup",
          "from": "thisYear", "key": "country",
          "fields": ["country"], "as": ["this"],
          "default": {}
        },
        {
          "type": "lookup",
          "from": "prevYear", "key": "country",
          "fields": ["country"], "as": ["prev"],
          "default": {}
        },
        {
          "type": "lookup",
          "from": "nextYear", "key": "country",
          "fields": ["country"], "as": ["next"],
          "default": {}
        },
        {
          "type": "formula",
          "as": "target_fertility",
          "expr": "interYear > currentYear ? datum.next.fertility : (datum.prev.fertility||datum.this.fertility)"
        },
        {
          "type": "formula",
          "as": "target_life_expect",
          "expr": "interYear > currentYear ? datum.next.life_expect : (datum.prev.life_expect||datum.this.life_expect)"
        },
        {
          "type": "formula",
          "as": "inter_fertility",
          "expr": "interYear==2000 ? datum.this.fertility : datum.this.fertility + (datum.target_fertility-datum.this.fertility) * abs(interYear-datum.this.year)/5"
        },
        {
          "type": "formula",
          "as": "inter_life_expect",
          "expr": "interYear==2000 ? datum.this.life_expect : datum.this.life_expect + (datum.target_life_expect-datum.this.life_expect) * abs(interYear-datum.this.year)/5"
        }
      ]
    },
    {
      "name": "trackCountries",
      "on": [
        {"trigger": "active", "toggle": "{country: active.country}"}
      ]
    }
  ],

  "signals": [
    { "name": "minYear", "value": 1955 },
    { "name": "maxYear", "value": 2005 },
    { "name": "stepYear", "value": 5 },
    {
      "name": "active",
      "value": {},
      "on": [
        {"events": "@point:pointerdown, @point:touchstart", "update": "datum"},
        {"events": "window:pointerup, window:touchend", "update": "{}"}
      ]
    },
    { "name": "isActive", "update": "active.country" },
    {
      "name": "timeline",
      "value": {},
      "on": [
        {"events": "@point:pointerover", "update": "isActive ? active : datum"},
        {"events": "@point:pointerout", "update": "active"},
        {"events": {"signal": "active"}, "update": "active"}
      ]
    },
    {
      "name": "tX",
      "on": [{
        "events": "pointermove!, touchmove!",
        "update": "isActive ? scale('x', active.this.fertility) : tX"
      }]
    },
    {
      "name": "tY",
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? scale('y', active.this.life_expect) : tY"
      }]
    },
    {
      "name": "pX",
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? scale('x', active.prev.fertility) : pX"
      }]
    },
    {
      "name": "pY",
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? scale('y', active.prev.life_expect) : pY"
      }]
    },
    {
      "name": "nX",
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? scale('x', active.next.fertility) : nX"
      }]
    },
    {
      "name": "nY",
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? scale('y', active.next.life_expect) : nY"
      }]
    },
    {
      "name": "thisDist",
      "value": 0,
      "on":[{
        "events": "pointermove, touchmove",
        "update": "isActive ? hypot(x()-tX, y()-tY) : thisDist"
      }]
    },
    {
      "name": "prevDist",
      "value": 0,
      "on":[{
        "events": "pointermove, touchmove",
        "update": "isActive ? hypot(x()-pX, y()-pY): prevDist"
      }]
    },
    {
      "name": "nextDist",
      "value": 0,
      "on":[{
        "events": "pointermove, touchmove",
        "update": "isActive ? hypot(x()-nX, y()-nY) : nextDist"
      }]
    },
    {
      "name": "prevScore",
      "value": 0,
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? ((pX-tX) * (x()-tX) + (pY-tY) * (y()-tY))/prevDist || -999999 : prevScore"
      }]
    },
    {
      "name": "nextScore",
      "value": 0,
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? ((nX-tX) * (x()-tX) + (nY-tY) * (y()-tY))/nextDist || -999999 : nextScore"
      }]
    },
    {
      "name": "interYear",
      "value": 1980,
      "on": [{
        "events": "pointermove, touchmove",
        "update": "isActive ? (min(maxYear, currentYear+5, max(minYear, currentYear-5, prevScore > nextScore ? (currentYear - 2.5*prevScore/hypot(pX-tX, pY-tY)) : (currentYear + 2.5*nextScore/hypot(nX-tX, nY-tY))))) : interYear"
      }]
    },
    {
      "name": "currentYear",
      "value": 1980,
      "on":[{
        "events": "pointermove, touchmove",
        "update": "isActive ? (min(maxYear, max(minYear, prevScore > nextScore ? (thisDist < prevDist ? currentYear : currentYear-5) : (thisDist < nextDist ? currentYear : currentYear+5)))) : currentYear"
      }]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "linear", "nice": true,
      "domain": {"data": "gapminder", "field": "fertility"},
      "range": "width"
    },
    {
      "name": "y",
      "type": "linear", "nice": true, "zero": false,
      "domain": {"data": "gapminder", "field": "life_expect"},
      "range": "height"
    },
    {
      "name": "color",
      "type": "ordinal",
      "domain": {"data": "gapminder", "field": "cluster"},
      "range": "category"
    },
    {
      "name": "label",
      "type": "ordinal",
      "domain": {"data": "clusters", "field": "id"},
      "range": {"data": "clusters", "field": "name"}
    }
  ],

  "axes": [
    {
      "orient": "bottom", "scale": "x",
      "domain": false, "ticks": false, "labels": false,
      "grid": true, "tickCount": 5, "zindex": 0
    },
    {
      "domain": false, "ticks": false, "labels": false,
      "orient": "left", "scale": "y",
      "grid": true, "tickCount": 5, "zindex": 0
    },
    {
      "title": "Fertility",
      "orient": "bottom", "scale": "x",
      "tickCount": 5, "zindex": 1
    },
    {
      "title": "Life Expectancy",
      "orient": "left", "scale": "y",
      "tickCount": 5, "zindex": 1
    }
  ],

  "legends": [
    {
      "fill": "color",
      "title": "Region",
      "orient": "right",
      "encode": {
        "symbols": {
          "enter": {
            "fillOpacity": {"value": 0.5}
          }
        },
        "labels": {
          "update": {
            "text": {"scale": "label", "field": "value"}
          }
        }
      }
    }
  ],

  "marks": [
    {
      "type": "text",
      "encode": {
        "update": {
          "text": {"signal": "currentYear"},
          "x": {"value": 300},
          "y": {"value": 300},
          "fill": {"value": "grey"},
          "fillOpacity": {"value": 0.25},
          "fontSize": {"value": 100}
        }
      }
    },
    {
      "type": "text",
      "from": {"data": "country_timeline"},
      "interactive": false,
      "encode": {
        "enter": {
          "x": {"scale": "x", "field": "fertility", "offset": 5},
          "y": {"scale": "y", "field": "life_expect"},
          "fill": {"value": "#555"},
          "fillOpacity": {"value": 0.6},
          "text": {"field": "year"}
        }
      }
    },
    {
      "type": "line",
      "from": {"data": "country_timeline"},
      "encode": {
        "update": {
          "x": {"scale": "x", "field": "fertility"},
          "y": {"scale": "y", "field": "life_expect"},
          "stroke": {"value": "#bbb"},
          "strokeWidth": {"value": 5},
          "strokeOpacity": {"value": 0.5}
        }
      }
    },
    {
      "name": "point",
      "type": "symbol",
      "from": {"data": "interpolate"},
      "encode": {
        "enter": {
          "fill": {"scale": "color", "field": "this.cluster"},
          "size": {"value": 150}
        },
        "update": {
          "x": {"scale": "x", "field": "inter_fertility"},
          "y": {"scale": "y", "field": "inter_life_expect"},
          "fillOpacity": [
            {
              "test": "datum.country==timeline.country || indata('trackCountries', 'country', datum.country)",
              "value": 1
            },
            {"value": 0.5}
          ]
        }
      }
    },
    {
      "type": "text",
      "from": {"data": "interpolate"},
      "interactive": false,
      "encode": {
        "enter": {
          "fill": {"value": "#333"},
          "fontSize": {"value": 14},
          "fontWeight": {"value": "bold"},
          "text": {"field": "country"},
          "align": {"value": "center"},
          "baseline": {"value": "bottom"}
        },
        "update": {
          "x": {"scale": "x", "field": "inter_fertility"},
          "y": {"scale": "y", "field": "inter_life_expect", "offset": -7},
          "fillOpacity": [
            {
              "test": "datum.country==timeline.country || indata('trackCountries', 'country', datum.country)",
              "value": 0.8
            },
            {"value": 0}
          ]
        }
      }
    }
  ]
}
