Add doc example of replacing tick labels with custom strings

[joelostblom]

Occasionally it is useful to att a string label to explain numerical tick values on an axis while still using the numerical value for calculating an average, e.g. in ratings or survey responses. I couldn't find an example of this in the docs, and only after much tinkering I was able to use labelExpr and tickCounts to figure out a way to do this. I am not sure if this is the most convenient way and maybe I missed a more convenient approach. One thing I have not been able to figure out is how to set an exact number of ticks instead of it being rounded to a nice value, but would you consider adding an example similar to this in the docs (I can PR if desired)?

Open the Chart in the Vega Editor

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.8.1.json",
  "data": {
    "name": "data-145f5f62f285527e098239bd826c1a90"
  },
  "datasets": {
    "data-145f5f62f285527e098239bd826c1a90": [
      {
        "IMDB Rating": 5.4,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 6.2,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 7.8,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 5.5,
        "Major Genre": "Comedy"
      },
      {
        "IMDB Rating": 5.5,
        "Major Genre": "Comedy"
      },
      {
        "IMDB Rating": 7.3,
        "Major Genre": "Comedy"
      },
      {
        "IMDB Rating": 5.3,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 5.4,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 8.9,
        "Major Genre": "Action"
      },
      {
        "IMDB Rating": 8.0,
        "Major Genre": "Action"
      }
    ]
  },
  "encoding": {
    "x": {
      "aggregate": "mean",
      "axis": {
        "labelExpr": "datum.label == 0 ? 'Poor' : datum.label == 5 ? 'Neutral' : 'Great'",
        "labelFlush": false,
        "tickCount": 3
      },
      "field": "IMDB Rating",
      "scale": {
        "domain": [
          0,
          10
        ]
      },
      "title": "",
      "type": "quantitative"
    },
    "y": {
      "field": "Major Genre",
      "sort": "x",
      "title": "",
      "type": "nominal"
    }
  },
  "mark": {
    "size": 80,
    "type": "circle"
  }
}

• Check for duplicate issues. Please file separate requests as separate issues on GitHub.
• Describe the feature's goal, motivating use cases, and its expected behavior.
• If you are proposing a new syntax, please provide at least one example spec, wrapped by triple backticks like this:
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[domoritz]

I like the example. Here is a slight cleanup (Open the Chart in the Vega Editor):

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "url": "data/movies.json"
  },
  "mark": {"size": 80, "type": "circle"},
  "encoding": {
    "x": {
      "aggregate": "mean",
      "axis": {
        "labelExpr": "datum.label == 0 ? 'Poor' : datum.label == 5 ? 'Neutral' : 'Great'",
        "labelFlush": false,
        "values": [0, 5, 10]
      },
      "field": "IMDB Rating",
      "scale": {"domain": [0, 10]},
      "title": null
    },
    "y": {"field": "Major Genre", "sort": "x", "title": null}
  }
}

Yes, please send a pull request!

[joelostblom]

Great, thank you for the cleanup @domoritz ! I opened PR #7047