Coloring by a temporal field doesn't use the full range of colors available

[nicolaskruchten]

Bug Description

This spec (editor link) shows that Vega-Lite (Vega?) doesn't seem to use the full range of the plasma color scheme when coloring by a temporal field which is a shame:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Stock prices of 5 Tech Companies over Time.",
  "data": {"url": "data/stocks.csv"},
  "mark": {"type": "point"},
  "encoding": {
    "x": {"timeUnit": "year", "field": "date"},
    "y": {"aggregate": "mean", "field": "price", "type": "quantitative"},
    "color": {
      "field": "date",
      "type": "temporal",
      "scale": {"scheme": "plasma"}
    }
  }
}

If applicable, include error messages and screenshots, videos, or working examples (e.g. by clicking share in the Vega-Editor.

Checklist

• I checked for duplicate issues.

[PBI-David]

This might be a Vega issue. Here is a corrected Vega spec changing the type to scale type to linear and the legend formatType to time.

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "Stock prices of 5 Tech Companies over Time.",
  "background": "white",
  "padding": 5,
  "width": 200,
  "height": 200,
  "style": "cell",
  "data": [
    {
      "name": "source_0",
      "url": "data/stocks.csv",
      "format": {"type": "csv", "parse": {"date": "date"}, "delimiter": ","},
      "transform": [
        {
          "field": "date",
          "type": "timeunit",
          "units": ["year"],
          "as": ["year_date", "year_date_end"]
        },
        {
          "type": "aggregate",
          "groupby": ["year_date", "date"],
          "ops": ["mean"],
          "fields": ["price"],
          "as": ["mean_price"]
        },
        {
          "type": "filter",
          "expr": "(isDate(datum[\"date\"]) || (isValid(datum[\"date\"]) && isFinite(+datum[\"date\"]))) && isValid(datum[\"mean_price\"]) && isFinite(+datum[\"mean_price\"])"
        }
      ]
    }
  ],
  "marks": [
    {
      "name": "marks",
      "type": "symbol",
      "style": ["point"],
      "from": {"data": "source_0"},
      "encode": {
        "update": {
          "fill": {"value": "transparent"},
          "stroke": {"scale": "color", "field": "date"},
          "ariaRoleDescription": {"value": "point"},
          "description": {
            "signal": "\"date (year): \" + (timeFormat(datum[\"year_date\"], timeUnitSpecifier([\"year\"], {\"year-month\":\"%b %Y \",\"year-month-date\":\"%b %d, %Y \"}))) + \"; Mean of price: \" + (format(datum[\"mean_price\"], \"\")) + \"; date: \" + (timeFormat(datum[\"date\"], '%b %d, %Y'))"
          },
          "x": {"scale": "x", "field": "year_date"},
          "y": {"scale": "y", "field": "mean_price"}
        }
      }
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "time",
      "domain": {"data": "source_0", "field": "year_date"},
      "range": [0, {"signal": "width"}]
    },
    {
      "name": "y",
      "type": "linear",
      "domain": {"data": "source_0", "field": "mean_price"},
      "range": [{"signal": "height"}, 0],
      "nice": true,
      "zero": true
    },
    {
      "name": "color",
      "type": "linear",
      "domain": {"data": "source_0", "field": "date" },
      "range": {"scheme": "plasma"}
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "gridScale": "y",
      "grid": true,
      "tickCount": {"signal": "ceil(width/40)"},
      "tickMinStep": {
        "signal": "datetime(2002, 0, 1, 0, 0, 0, 0) - datetime(2001, 0, 1, 0, 0, 0, 0)"
      },
      "domain": false,
      "labels": false,
      "aria": false,
      "maxExtent": 0,
      "minExtent": 0,
      "ticks": false,
      "zindex": 0
    },
    {
      "scale": "y",
      "orient": "left",
      "gridScale": "x",
      "grid": true,
      "tickCount": {"signal": "ceil(height/40)"},
      "domain": false,
      "labels": false,
      "aria": false,
      "maxExtent": 0,
      "minExtent": 0,
      "ticks": false,
      "zindex": 0
    },
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "date (year)",
      "format": {
        "signal": "timeUnitSpecifier([\"year\"], {\"year-month\":\"%b %Y \",\"year-month-date\":\"%b %d, %Y \"})"
      },
      "labelFlush": true,
      "labelOverlap": true,
      "tickCount": {"signal": "ceil(width/40)"},
      "tickMinStep": {
        "signal": "datetime(2002, 0, 1, 0, 0, 0, 0) - datetime(2001, 0, 1, 0, 0, 0, 0)"
      },
      "zindex": 0
    },
    {
      "scale": "y",
      "orient": "left",
      "grid": false,
      "title": "Mean of price",
      "labelOverlap": true,
      "tickCount": {"signal": "ceil(height/40)"},
      "zindex": 0
    }
  ],
  "legends": [
    {
      "stroke": "color",
      "format": "%b %d, %Y",  "formatType":"time",
      
      "title": "date"
    }
  ]
}

[nicolaskruchten]

Thanks! But if Vega is able to do this, it seems like Vega-Lite could be modified/fixed to output this kind of thing, right?

OTOH, looking at the compiled Vega for my example, it seems like it's just getting the range wrong.

[PBI-David]

Yes, here is the VL. The reason I said I thought it was a Vega issue is the range is being computed incorrectly by Vega when the type is time.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Stock prices of 5 Tech Companies over Time.",
  "data": {"url": "data/stocks.csv"},
  "mark": {"type": "point"},
  "encoding": {
    "x": {"timeUnit": "year", "field": "date"},
    "y": {"aggregate": "mean", "field": "price", "type": "quantitative"},
    "color": {
      "field": "date",
      "type": "quantitative",
      "scale": {"scheme": "plasma"}, "legend":{"formatType": "time"}
    }
  }
}

[nicolaskruchten]

Ah I see, thanks for that. Should I file a bug in the Vega repo?

[PBI-David]

I would do as they might know why the range is being calculated incorrectly.