Points only in the central line

I am using this example named "Line Chart with Point Markers" as reference, but not see other example or any clues about conditional or "selected by symbol" points.

The illustration shows a typical case (see also SPC) where I need only the blue central line with dots.

Solution

You can do this by layering filtered versions of the dataset. Modifying the example you linked to, it might look something like this (vega editor):

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "description": "Stock prices of 5 Tech Companies over Time.",
  "data": {"url": "data/stocks.csv"},
  "encoding": {
    "x": {"timeUnit": "year", "field": "date", "type": "temporal"},
    "y": {"aggregate": "mean", "field": "price", "type": "quantitative"},
    "color": {"field": "symbol", "type": "nominal"}
  },
  "layer": [
    {
      "mark": {"type": "line", "point": true},
      "transform": [{"filter": "datum.symbol == 'GOOG'"}]
    },
    {
      "mark": {"type": "line"},
      "transform": [{"filter": "datum.symbol != 'GOOG'"}]
    }
  ]
}