What you're trying to do isn't logically possible. You have faceted on country. That means each country gets a line (working), a symbol (working) but how would it get a rule where the start point is one country and the end point is the other. One group does not know anything about the other group so you couldn't have a rule spanning the two countries.
If you want to keep your facet, you can do the following:
{
"$schema": "https://vega.github.io/schema/vega/v5.json",
"description": "A basic line chart example.",
"width": 500,
"height": 200,
"padding": 5,
"signals": [],
"data": [
{
"name": "bombers",
"url": "https://gist.githubusercontent.com/vdvoorder/5b30997d8708dda783bb2b95d2e9ef34/raw/Bomber%2520Gap.csv",
"format": {
"type": "csv",
"parse": {"Total bombers": "number", "Year": "number"}
},
"transform": [
{
"type": "formula",
"as": "date",
"expr": "time(datetime(datum.Year, 1, 1))"
},
{"type": "filter", "expr": "datum.Year <= 1980"}
]
},
{
"name": "bombers2",
"source": "bombers",
"transform": [
{
"type": "pivot",
"field": "Country",
"value": "Total bombers",
"groupby": ["date"]
}
]
}
],
"scales": [
{
"name": "x",
"type": "time",
"range": "width",
"domain": {"data": "bombers", "field": "date"}
},
{
"name": "y",
"type": "linear",
"range": "height",
"nice": true,
"zero": true,
"domain": {"data": "bombers", "field": "Total bombers"}
},
{
"name": "color",
"type": "ordinal",
"range": "category",
"domain": {"data": "bombers", "field": "Country"}
}
],
"axes": [
{"orient": "bottom", "scale": "x"},
{"orient": "left", "scale": "y"}
],
"marks": [
{
"type": "group",
"from": {
"facet": {"name": "series", "data": "bombers", "groupby": "Country"}
},
"marks": [
{
"description": "Line for evolution of total bombers by country",
"type": "line",
"from": {"data": "series"},
"encode": {
"enter": {
"x": {"scale": "x", "field": "date"},
"y": {"scale": "y", "field": "Total bombers"},
"stroke": {"scale": "color", "field": "Country"},
"strokeCap": {"value": "round"},
"strokeWidth": {"value": 3},
"strokeOpacity": {"value": 0.5}
}
}
},
{
"description": "Points for total bombers by country",
"type": "symbol",
"from": {"data": "series"},
"encode": {
"enter": {
"x": {"scale": "x", "field": "date"},
"y": {"scale": "y", "field": "Total bombers"},
"size": {"value": 50},
"fill": {"scale": "color", "field": "Country"},
"strokeWidth": {"value": 20},
"stroke": {"value": "lightskyblue"}
},
"update": {
"fillOpacity": {"value": 1},
"strokeOpacity": {"value": 0}
},
"hover": {"strokeOpacity": {"value": 1}}
}
}
]
},
{
"description": "Rulers between country total bomber numbers",
"type": "rule",
"from": {"data": "bombers2"},
"encode": {
"update": {
"x": {"scale": "x", "field": "date"},
"y": {"scale": "y", "field": "Soviet Union"},
"y2": {"scale": "y", "field": "United States"},
"stroke": {"value": "lightskyblue"},
"strokeWidth": {"value": 3},
"strokeOpacity": {"value": 0.5}
}
}
}
]
}
