Window smoothing with filter

spec.json

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {"sequence": {"start": 0, "stop": 100, "as": "x"}},
  "transform": [{"calculate": "random() * sin(datum.x/10)", "as": "y"}],
  "vconcat": [
    {
      "width": 600,
      "mark": "line",
      "encoding": {
        "x": {"field": "x", "type": "quantitative"},
        "y": {"field": "y", "type": "quantitative"}
      }
    },
    {
      "width": 600,
      "transform": [
        {"filter": "datum.x < end"},
        {
          "window": [{"field": "y", "op": "mean", "as": "mean"}],
          "frame": [-10, 10]
        }
      ],
      "mark": "line",
      "encoding": {
        "x": {
          "field": "x",
          "type": "quantitative",
          "scale": {"domain": [0, 100]}
        },
        "y": {"field": "mean", "type": "quantitative"}
      }
    }
  ],
  "config": {
    "signals": [{"name": "end", "value": 50, "bind": {"type": "range"}}]
  }
}