dynamic field sort

dynamic_field_sort.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"a": "A", "b": 28},
      {"a": "B", "b": 55},
      {"a": "C", "b": 43},
      {"a": "D", "b": 91},
      {"a": "E", "b": 81},
      {"a": "F", "b": 53},
      {"a": "G", "b": 19},
      {"a": "H", "b": 87},
      {"a": "I", "b": 52}
    ]
  },
  "params": [
    {
      "name": "sortChoice",
      "value": "x",
      "bind": {"input": "select", "options": ["x", "y", "-x", "-y"]}
    },
    {
      "name": "sortBy",
      "expr": "sortChoice==='x'|| sortChoice==='-x' ? 'a':'b'"
    },
    {
      "name": "sortOrder",
      "expr": "sortChoice==='x'|| sortChoice==='y' ? 'ascending':'descending'"
    }
  ],
  "transform": [
    {
      "sort": [
        {"field": {"signal": "sortBy"}, "order": {"signal": "sortOrder"}}
      ],
      "window": [{"op": "rank", "as": "sorted"}]
    }
  ],
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "sort": {"field": "sorted"}},
    "y": {"field": "b", "type": "quantitative"}
  }
}