Line overlay only shows one line for ranged area

[kanitw]

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "title": "Seattle Temperature Range, 2012-2015",
  "data": {"url": "data/seattle-weather.csv"},
  "width": 600,
  "height": 300,
  "mark": {"type": "area", "line": true},
  "encoding": {
    "x": {
      "axis": {"format": "%b"}, 
      "title": "Date",
      "field": "date",
      "timeUnit": "month"
    }, 
    "y": {
      "title": "Daily Temperature Range (C)",
      "aggregate": "max",
      "field": "temp_max"
    },
    "y2": {"aggregate": "min", "field": "temp_min"},
    "opacity": {"value": 0.7}
  }
}

Currently it looks like this.

With #8472 (review), it will show one line at the top, instead of two.