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

Question

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.

Answer 1

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": {...}
}