scatter plot of diabetes patients with healthy area

spec.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "scatter plot of diabetes patients",
  "width": 500,
  "data": {
    "values": [
      {"sex": "male", "glucose": 190, "bmi": 28},
      {"sex": "male", "glucose": 95, "bmi": 24},
      {"sex": "male", "glucose": 150, "bmi": 34},
      {"sex": "female", "glucose": 80, "bmi": 28},
      {"sex": "female", "glucose": 140, "bmi": 31},
      {"sex": "female", "glucose": 120, "bmi": 16},
      {"sex": "female", "glucose": 130, "bmi": 22}
    ]
  },
  "layer": [
    {
      "mark": {
        "type": "rect",
          "fillOpacity": 0.1
      },
      "encoding": {
        "x": {"datum": 60 },
        "x2": {"datum": 120 },
        "y": {"datum": 18.5 },
        "y2": {"datum": 25 }
      }
    },
    {
      "mark": "circle",
      "encoding": {
        "x": {"field": "glucose", "type": "quantitative"},
        "y": {"field": "bmi", "type": "quantitative"}
      }
    }
  ]
}