How to draw vertical lines based on a different quantity than that for the x channel in charts by row channel?

I'd like to have multiple time series drawn by row channel ("field": "PLATFORM"), x channel: ("field": "estimating-date-time"), and y channel ("field": "eta-variance"). Besides the lines of time series, I'd like to draw a vertical line at x = arrvial-time which is another field, conditioned by the value of "PLATFORM". The following is a working example of the charts except the desirable vertical line in each chart: vega-lite for multiple time series

Below is the desired effect with manual illustration:

My question is how to add the vertical line for each chart to the specifications?

The challenge to me is that the field "arrival-time" from which the value used to draw the vertical line is not the same as the chart's x channel "estimating-data-time". I've found examples of drawing such a line using a value related to the same x channel.

Solution

You can do this by nesting a layer specification within a facet operator; something like this (open in editor):

{
  "facet": {"row": {"field": "PLATFORM"}},
  "spec": {
    "height": 80,
    "width": 300,
    "layer": [
      {
        "mark": "line",
        "encoding": {
          "x": {"field": "estimating-date-time", "type": "temporal"},
          "y": {"field": "ETA-variance", "type": "quantitative"}
        }
      },
      {
        "mark": "rule",
        "encoding": {"x": {"field": "arrival-time", "type": "temporal"}}
      }
    ]
  },
  "data": {...}
}