vega multiple time series + transform

example.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json", 
  "description": "Multi-series Line Chart",
  "data": {
    "values": [
      { "variable": "beta", "sample": 0, "time": 1, "value": 1 },
      { "variable": "beta", "sample": 0, "time": 2, "value": 2 },
      { "variable": "beta", "sample": 0, "time": 3, "value": 3 },
      { "variable": "beta", "sample": 0, "time": 4, "value": 3 },

      { "variable": "gamma", "sample": 0, "time": 1, "value": 1 },
      { "variable": "gamma", "sample": 0, "time": 2, "value": 0 },
      { "variable": "gamma", "sample": 0, "time": 3, "value": 1 },
      { "variable": "gamma", "sample": 0, "time": 4, "value": 1 },

      { "variable": "gamma", "sample": 1, "time": 1, "value": 0 },
      { "variable": "gamma", "sample": 1, "time": 2, "value": 2 },
      { "variable": "gamma", "sample": 1, "time": 3, "value": 4 },
      { "variable": "gamma", "sample": 1, "time": 4, "value": 2 },
    ]
  }, 
  "transform": [
    {
      "calculate": "datum.variable + datum.sample",
      "as": "key"
    },
    {
      "pivot": "key",
      "value": "value",
      "groupby": ["time"]
    }
  ],
  "repeat": {
    "layer": ["beta0", "gamma0", "gamma1"]
  },
  "spec": {
    "layer": [
      {
        "mark": {"type": "line", "stroke": "white", "strokeWidth": 4},
        "encoding": {
          "x": {"field": "time", "type": "quantitative"},
          "y": {"field": {"repeat": "layer"}, "type": "quantitative", "title": "y-axis"},
          // "stroke": {"datum": {"repeat": "layer"}, "type": "nominal"},
          "stroke": {"datum": {"repeat": "layer"}, "type": "nominal"}
        }
      }
    ]
  }
}