Line Chart with Markers and Invalid Values

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {
        "x": 1,
        "y": 10
      },
      {
        "x": 2,
        "y": 30
      },
      {
        "x": 3,
        "y": null
      },
      {
        "x": 4,
        "y": 15
      },
      {
        "x": 5,
        "y": null
      },
      {
        "x": 6,
        "y": 40
      },
      {
        "x": 7,
        "y": 20
      }
    ]
  },
  "mark": {"type": "line", "point": true},
  "encoding": {
    "x": {"field": "x", "type": "quantitative"},
    "y": {"field": "y", "type": "quantitative"}
  }
}