stack bar with rule (Vega-Lite spec from Tue Apr 13 2021)

spec.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "width": 500,
  "layer": [
    {
      "mark": {"type": "bar", "tooltip": true},
      "encoding": {
        "color": {"field": "supertype"},
        "y": {"field": "sum", "type": "quantitative", "scale": {"padding": 1}},
        "x": {
          "field": "date",
          "type": "temporal",
          "timeUnit": "month",
          "scale": {"padding": 0}
        }
      }
    },
    {
      "mark": "rule",
      "encoding": {
        "x": {
          "field": "date",
          "type": "temporal",
          "timeUnit": "month",
          "bandPosition": 0.5
        },
        "y": {}
      }
    }
  ],
  "data": {
    "values": [
      {"date": "2020-01-01", "supertype": "income", "sum": 500},
      {"date": "2020-02-01", "supertype": "income", "sum": 200},
      {"date": "2020-03-01", "supertype": "income", "sum": 400},
      {"date": "2020-08-01", "supertype": "income", "sum": 200},
      {"date": "2020-09-01", "supertype": "income", "sum": 300},
      {"date": "2020-10-01", "supertype": "income", "sum": 330},
      {"date": "2020-01-01", "supertype": "expense", "sum": -100},
      {"date": "2020-02-01", "supertype": "expense", "sum": -250},
      {"date": "2020-03-01", "supertype": "expense", "sum": -300},
      {"date": "2020-08-01", "supertype": "expense", "sum": -500},
      {"date": "2020-09-01", "supertype": "expense", "sum": -800},
      {"date": "2020-10-01", "supertype": "expense", "sum": -900}
    ]
  }
}