How to specify the order of labels in the color legend of Vega?

Question

I have the following definition which draws lines from two groups of data: A and B. When using the "color" property in "encoding", Vega will create a legend for the colors and will display the groups in alphabetical order by default. How can I specify the that I want B to appear before A in the legend?

More generally, I want to be able to sort labels in any order, not just descending or ascending order.

See the chart here.

{
  "data": {
    "values": [
      {"x":"1", "group": "B", "value":0.2},
      {"x":"2", "group": "B", "value":0.4},
      {"x":"3", "group": "B", "value":0.3},
      {"x":"1", "group": "A", "value":0.1},
      {"x":"2", "group": "A", "value":0},
      {"x":"3", "group": "A", "value":0.2}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "x"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "group"}
  }
}

Answer 1

You can add a sort to the encoding.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  
  "data": {
    "values": [
      {"x": "1", "group": "B", "value": 0.2},
      {"x": "2", "group": "B", "value": 0.4},
      {"x": "3", "group": "B", "value": 0.3},
      {"x": "1", "group": "A", "value": 0.1},
      {"x": "2", "group": "A", "value": 0},
      {"x": "3", "group": "A", "value": 0.2}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {"field": "x"},
    "y": {"field": "value", "type": "quantitative"},
    "color": {"field": "group", "sort": "descending"}
  }

}

You can also add a custom sort as follows:

"sort": ["B", "A", "C"]