Vega-Lite Chart with vertical rule movable with a slider.

spec.json

/* Layering a verticle rule over existing data (with a control to boot) */
{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
        "params": [
        {
          "name": "x1",
          "value": 5,
          "bind": {"input": "range", "min": 0, "max": 10, "step": 1}
        }
      ],
      "layer": [
    {
      "data": {
        "values": [
          {"a": 1, "b": 30},
          {"a": 2, "b": 45},
          {"a": 3, "b": 50},
          {"a": 4, "b": 60},
          {"a": 5, "b": 70},
          {"a": 6, "b": 80},
          {"a": 7, "b": 70},
          {"a": 8, "b": 50},
          {"a": 9, "b": 10}
        ]
      },
      "mark": {"type":"line", "size": 2},
      "encoding": {
        "x": {"field": "a", "type": "quantitative"},
        "y": {"field": "b", "type": "quantitative"}
      }
    },
    {
      "data": {"values": [{}]},
      "mark": {"type": "rule", "strokeDash": [4, 4], "size": 2, "color": "grey"},
      "encoding": {
        "x": {"datum": {"expr": "x1"}, "type": "quantitative", "scale": {"domain": [0,10]}}
      }
    }
  ]
}

https://vega.github.io/editor/#/gist/09f1635e965ea862f229184a717b82b2/spec.json