Vega Lite - scatter plot

Find the max x and y values in your data, take the largest, and then add a layer that draws from 0 to that point.

Here is a simplified version:

{
  "layer": [
    {
      "data": {"name": "dataset"},
      "mark": "point",
      "encoding": {
        "y": {
          "field": "y",
          "type": "quantitative"
        },
        "x": {
          "field": "x",
          "type": "quantitative"
        }
      }
    },
    {
      "data": {
        "values": [
          {"x": 0, "y": 0},
          {"x": 4, "y": 4}
        ]
      },
      "mark": {
        "type": "line",
        "from": {"data": "data2"}
      },
      "encoding": {
        "y": {
          "field": "y",
          "type": "quantitative"
        },
        "x": {
          "field": "x",
          "type": "quantitative"
        }
      }
    }
  ]
}