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vega-lite

How to avoid zoom conflict in a line chart with layer?

VEGA-lite is not perfect, but is very good, and in general for something that looks like a bug, there are a workaround... So I supposing that in this "bug" we have a workaround.

((edit after answer: it is not a real bug, is a "semantic bug" on the specification language))

The strange behaviour, a "semantic bug": I was using selection: { "grid": {"type":"interval", "bind":"scales"} } for zoom, in a trivial context, with simple mark: 'line'. When I add layer, it stopts to work.

    {
        title: "Número de registros por minuto (n_count normalizado)", 
        $schema: vglVers,
        data: { "url":"mySQLtable" },
        selection: { "grid": {"type":"interval", "bind":"scales"} }, // was working with simple mark
        //mark: 'line',
        width:340,
        encoding: {
          x: {"field": "instant", "type": "temporal"},
          y: {"field": "n_pmin", "type": "quantitative"},
          color: {"field": "symbol", "type": "nominal"}
        },
        layer: [
            {
              "mark": {"type": "line", "point": true},
              "transform": [{"filter": "datum.symbol == 'n_pmin'"}]
            },
            { "mark": {"type": "line"}, "transform": [{"filter": "datum.symbol != 'n_pmin'"}]  }
        ]
      }

The workaround: as @jakevdp commented here, "the interval selection must be added to one of the layers". But

  1. How to do this "interval selection"?

  2. The data on my chart is not static, I need a interval that changes with it, so, not make sense to set a interval.

Solution

  • The "interval selection" I referred to is the interval selection definition within your chart:

    selection: { "grid": {"type":"interval", "bind":"scales"} }

    You cannot declare it in the top-level chart; you must declare it in one of the layers:

    {
    title: "Número de registros por minuto (n_count normalizado)", 
    $schema: vglVers,
    data: { "url":"mySQLtable" },
    width:340,
    encoding: {
      x: {"field": "instant", "type": "temporal"},
      y: {"field": "n_pmin", "type": "quantitative"},
      color: {"field": "symbol", "type": "nominal"}
    },
    layer: [
        {
          "mark": {"type": "line", "point": true},
          "transform": [{"filter": "datum.symbol == 'n_pmin'"}],
          "selection": {"grid": {"type":"interval", "bind":"scales"}},
        },
        {
          "mark": {"type": "line"},
          "transform": [{"filter": "datum.symbol != 'n_pmin'"}]
        }
    ]
 }

    Your issue is not a bug, nor is my solution a workaround: the vega-lite schema specifies that selections must be declared within a unit spec (i.e. an individual layer).