https://stackoverflow.com/questions/53673056/mouse-interaction-for-dragging-data-points-in-vega

spec.json

{
  "$schema": "https://vega.github.io/schema/vega/v4.json",
  "width": 500,
  "height": 200,
  "autosize": "none",
  "signals": [
    {
      "name": "whichPoint",
      "on": [
        {
          "events": "path:click, path:mousemove[event.buttons]{20}",
          "update": "datum.datum"
        }
      ]
    },
    {
      "name": "newPointPosition",
      "on": [
        {
          "events": "path:click, path:mousemove[event.buttons]{20}",
          "update": "{u: invert('xscale', x()), v: invert('yscale', y())}"
        }
      ]
    }
  ],
  "data": [
    {
      "name": "table",
      "values": [
        {"u": 0.1, "v": 0.1},
        {"u": 0.9, "v": 0.1},
        {"u": 0.1, "v": 0.9},
        {"u": 0.9, "v": 0.9},
        {"u": 0.5, "v": 0.5}
      ],
      "on": [
        {
          "trigger": "whichPoint",
          "modify": "whichPoint",
          "values": "{u: newPointPosition.u, v:newPointPosition.v}"
        }
      ]
    }
  ],
  "scales": [
    {"name": "xscale", "domain": [0, 1], "range": "width"},
    {"name": "yscale", "domain": [0, 1], "range": "height"}
  ],
  "marks": [
    {
      "name": "points",
      "type": "symbol",
      "zindex": 1,
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "fill": {"value": "black"},
          "size": {"value": 36},
          "x": {"scale": "xscale", "field": "u"},
          "y": {"scale": "yscale", "field": "v"}
        },
        "update": {
          "fill": {"value": "black"},
          "size": {"value": 36},
          "x": {"scale": "xscale", "field": "u"},
          "y": {"scale": "yscale", "field": "v"}
        }
      }
    },
    {
      "type": "path",
      "from": {"data": "points"},
      "encode": {
        "enter": {
          "stroke": {"value": "firebrick"},
          "fill": {"value": "transparent"}
        },
        "update": {
          "stroke": {"value": "firebrick"},
          "fill": {"value": "transparent"}
        }
      },
      "transform": [
        {
          "type": "voronoi",
          "x": "datum.x",
          "y": "datum.y",
          "size": [{"signal": "width"}, {"signal": "height"}]
        }
      ]
    }
  ]
}