Math — Chart: Frequency Polygon

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Frequency polygon",
  "data": {"values": [
    {"mid": 5, "freq": 2},
    {"mid": 15, "freq": 7},
    {"mid": 25, "freq": 14},
    {"mid": 35, "freq": 20},
    {"mid": 45, "freq": 13},
    {"mid": 55, "freq": 6},
    {"mid": 65, "freq": 2}
  ]},
  "mark": {"type": "line", "point": true},
  "encoding": {
    "x": {"field": "mid", "type": "quantitative", "title": "class midpoint"},
    "y": {"field": "freq", "type": "quantitative", "title": "frequency"}
  }
}