如何在Vega Lite中为单系列或多系列图表添加图例？

{
  "data": {
    "values": [
      {"x": "A", "y": 28},
      {"x": "B", "y": 55},
      {"x": "C", "y": 43}
    ]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "x", "type": "ordinal"},
    "y": {"field": "y", "type": "quantitative"}
  },
  "legend": true
}

{
  "data": {
    "values": [
      {"x": "A", "y": 28, "series": "Series 1"},
      {"x": "B", "y": 55, "series": "Series 1"},
      {"x": "C", "y": 43, "series": "Series 2"}
    ]
  },
  "mark": "point",
  "encoding": {
    "x": {"field": "x", "type": "ordinal"},
    "y": {"field": "y", "type": "quantitative"},
    "color": {"field": "series", "type": "nominal"}
  },
  "legend": {
    "title": "Series",
    "orient": "top-right"
  }
}