Vega-Lite Wilkinson Dot Plot, group by first digit

Question

I see the code to create a Wilkinson Dot Plot in Vega-Lite:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "A Wilkinson Dot Plot",
  "height": 100,
  "data": {
    "values": [
      10,11,11,11,14,15,17,
      22,25,26,28,
      33,33,33,34,37
    ]
  },
  "transform": [{
    "window": [{"op": "rank", "as": "id"}],
    "groupby": ["data"]
  }],
  "mark": {
    "type": "circle",
    "opacity": 1
  },
  "encoding": {
    "x": {"field": "data", "type": "ordinal"},
    "y": {"field": "id", "type": "ordinal", "axis": null, "sort": "descending"}
  }
}

Creates a plot grouping the exact numbers, but I'd like the output to be by the first digit so really theres 7 vertical dots for 1, 4 vertical dots for 2, and 5 vertical dots for 3. I tried adding calculation: "str.map(x => x.charAt(0))" to the transform array so I could group by that, but was unsuccessful in my execution. Any ideas appreciated!

Answer 1

You were on the right track, except that calculate transforms cannot use arbitrary javascript code, but only the subset made available in Vega Expressions . So, for example, you could do something like this ( vega editor ):

{
  "data": {
    "values": [10, 11, 11, 11, 14, 15, 17, 22, 25, 26, 28, 33, 33, 33, 34, 37]
  },
  "transform": [
    {"calculate": "floor(datum.data / 10)", "as": "data"},
    {
      "window": [{"op": "rank", "field": "data", "as": "id"}],
      "groupby": ["data"]
    }
  ],
  "mark": {"type": "circle", "opacity": 1},
  "encoding": {
    "x": {"field": "data", "type": "ordinal"},
    "y": {"field": "id", "type": "ordinal", "axis": null, "sort": "descending"}
  }
}