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Json – Vega-lite Gradient: Specify Where the Color Changes Relative to the Axis Scale

TylerRinker
May 30, 2024
119 views
0 votes
2 Answers

I have a metric and want to show a gradient color in an area chart corresponding to good (0 to .1 = green), ok (>.1 to .3 = yellow), bad (> .3 = red) for that metric at various time points. How an I make the color cut points relative to the actual scale on the y axis?

I thought I could specify the color breaks with offset and it looked correct with my first pass:

{
  "width": 600,
  "data": {
    "values": [
      {
        "__row__": 0,
        "calendar_month_year": "Dec-23",
        "concentration_risk_monthly": 0.51
      },
      {
        "__row__": 1,
        "calendar_month_year": "Jan-24",
        "concentration_risk_monthly": 0.5
      },
      {
        "__row__": 2,
        "calendar_month_year": "Feb-24",
        "concentration_risk_monthly": 0.27
      },
      {
        "__row__": 3,
        "calendar_month_year": "Mar-24",
        "concentration_risk_monthly": 0.22
      },
      {
        "__row__": 4,
        "calendar_month_year": "Apr-24",
        "concentration_risk_monthly": 0.25
      },
      {
        "__row__": 5,
        "calendar_month_year": "May-24",
        "concentration_risk_monthly": 0.22
      }
    ]
  },
  "mark": {
    "type": "area",
    "color": {
      "x1": 1,
      "y1": 1,
      "x2": 1,
      "y2": 0,
      "gradient": "linear",
      "stops": [
        {"offset": 0, "color": "#00800166"},
        {"offset": 0.3, "color": "#F7B50066"},
        {"offset": 1, "color": "#A9281F66"}
      ]
    }
  },
  "encoding": {
    "x": {
      "field": "calendar_month_year",
      "type": "ordinal",
      "sort": {"field": "__row__"},
      "axis": {"title": null},
      "scale": {"padding": 0}
    },
    "y": {
      "field": "concentration_risk_monthly",
      "type": "quantitative",
      "axis": {"title": null}
    }
  }
}

Gives:

But if I change the data to be below .05 for that metric (well below the .1 for green):

{
  "width": 600,
  "data": {
    "values": [
      {
        "__row__": 0,
        "calendar_month_year": "Dec-23",
        "concentration_risk_monthly": 0.051
      },
      {
        "__row__": 1,
        "calendar_month_year": "Jan-24",
        "concentration_risk_monthly": 0.05
      },
      {
        "__row__": 2,
        "calendar_month_year": "Feb-24",
        "concentration_risk_monthly": 0.027
      },
      {
        "__row__": 3,
        "calendar_month_year": "Mar-24",
        "concentration_risk_monthly": 0.022
      },
      {
        "__row__": 4,
        "calendar_month_year": "Apr-24",
        "concentration_risk_monthly": 0.025
      },
      {
        "__row__": 5,
        "calendar_month_year": "May-24",
        "concentration_risk_monthly": 0.022
      }
    ]
  },
  "mark": {
    "type": "area",
    "color": {
      "x1": 1,
      "y1": 1,
      "x2": 1,
      "y2": 0,
      "gradient": "linear",
      "stops": [
        {"offset": 0.0, "color": "#00800166"},
        {"offset": 0.3, "color": "#F7B50066"},
        {"offset": 1.0, "color": "#A9281F66"}
      ]
    }
  },
  "encoding": {
    "x": {
      "field": "calendar_month_year",
      "type": "ordinal",
      "sort": {"field": "__row__"},
      "axis": {"title": null},
      "scale": {"padding": 0}
    },
    "y": {
      "field": "concentration_risk_monthly",
      "type": "quantitative",
      "axis": {"title": null}
    }
  }
}

I’d expect the entire area to be green but it seems that the offsets are relative to the overall area and now there are still red areas even though all metrics are below .1:

Answers

Try this. It isn’t working 100% and is very clunky but might be good enough for your use case.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "width": 600,
  "data": {
    "values": [
      {
        "__row__": 0,
        "calendar_month_year": "Dec-23",
        "concentration_risk_monthly": 0.051
      },
      {
        "__row__": 1,
        "calendar_month_year": "Jan-24",
        "concentration_risk_monthly": 0.05
      },
      {
        "__row__": 2,
        "calendar_month_year": "Feb-24",
        "concentration_risk_monthly": 0.027
      },
      {
        "__row__": 3,
        "calendar_month_year": "Mar-24",
        "concentration_risk_monthly": 0.022
      },
      {
        "__row__": 4,
        "calendar_month_year": "Apr-24",
        "concentration_risk_monthly": 0.025
      },
      {
        "__row__": 5,
        "calendar_month_year": "May-24",
        "concentration_risk_monthly": 0.022
      }
    ]
  },
  "transform": [{"extent": "concentration_risk_monthly", "param": "myExtent"}],
  "mark": {
    "type": "area",
    "fill": {
      "expr": "{'gradient': 'linear', 'x1':0,'y1':1,'x2':0,'y2':0,  'stops': [{'offset': 0, 'color': scale('stroke',  myExtent[0]>1?1:myExtent[0]>0.3?0.3:0)},{'offset': 0.3, 'color':  scale('stroke', myExtent[1]>1?1:myExtent[1]>0.3?0.3:0)},{'offset': 1, 'color':  scale('stroke', myExtent[1]>=1?1:myExtent[1]>0.3?0.3:0)}]}"
    },
    "strokeOpacity": 0
  },
  "encoding": {
    "stroke": {
      "field": "1",
      "legend": null,
      "scale": {
        "type": "linear",
        "range": ["green", "yellow", "red"],
        "domain": [0, 0.3, 1]
      }
    },
    "x": {
      "field": "calendar_month_year",
      "type": "ordinal",
      "sort": {"field": "__row__"},
      "axis": {"title": null},
      "scale": {"padding": 0}
    },
    "y": {
      "field": "concentration_risk_monthly",
      "type": "quantitative",
      "axis": {"title": null}
    }
  }
}

Just a very minor tweak to Davide’s answer.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "width": 600,
  "data": {
    "values": [
      {
        "__row__": 0,
        "calendar_month_year": "Dec-23",
        "concentration_risk_monthly": 0.8
      },
      {
        "__row__": 1,
        "calendar_month_year": "Jan-24",
        "concentration_risk_monthly": 0.3
      },
      {
        "__row__": 2,
        "calendar_month_year": "Feb-24",
        "concentration_risk_monthly": 0.027
      },
      {
        "__row__": 3,
        "calendar_month_year": "Mar-24",
        "concentration_risk_monthly": 0.022
      },
      {
        "__row__": 4,
        "calendar_month_year": "Apr-24",
        "concentration_risk_monthly": 0.125
      },
      {
        "__row__": 5,
        "calendar_month_year": "May-24",
        "concentration_risk_monthly": 0.022
      }
    ]
  },
  "transform": [{"extent": "concentration_risk_monthly", "param": "myExtent"}],
  "mark": {
    "type": "area",
    "fill": {
      "expr": "{'gradient': 'linear', 'x1':0,'y1':1,'x2':0,'y2':0,  'stops': [{'offset': 0, 'color': scale('stroke',  myExtent[0]>1?1:myExtent[0]>0.3?0.3:0)},{'offset': 0.3, 'color':  scale('stroke', myExtent[1]>1?1:myExtent[1]>0.3?0.3:0)},{'offset': 1, 'color':  scale('stroke', myExtent[1]>=1?1:myExtent[1]>0.3?1:0.3)}]}"
    },
    "strokeOpacity": 0
  },
  "encoding": {
    "stroke": {
      "field": "1",
      "legend": null,
      "scale": {
        "type": "linear",
        "range": ["#00800166", "#F7B50066", "#A9281F66"],
        "domain": [0, 0.3, 1]
      }
    },
    "x": {
      "field": "calendar_month_year",
      "type": "ordinal",
      "sort": {"field": "__row__"},
      "axis": {"title": null},
      "scale": {"padding": 0}
    },
    "y": {
      "field": "concentration_risk_monthly",
      "type": "quantitative",
      "axis": {"title": null}
    }
  }
}

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