vega 3.0 dataflow event stream outside of vega

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://unpkg.com/d3"></script>
  <script src="https://unpkg.com/vega@3.0.2/build/vega-core.min.js"></script>
</head>

<body>
  <div id = "vis"></div>
  <pre id = "pre-hovered"></pre>
  <script>
		// spec from vega 3.0 examples
    //  https://github.com/uwdata/vega/blob/master/test/spec/bar-hover-label.vg.json
    var spec = {
      "width": 400,
      "height": 200,
      "padding": 5,

"signals": [
        {
          "name": "label",
          "value": {"x": 0, "y": 0},
          "on": [
            {"events": "rect:mouseover", "update": "datum"},
            {"events": "rect:mouseout", "update": "{x:0, y:0}"}
          ]
        }
      ],

"data": [
        {
          "name": "table",
          "values": [
            {"x": 1,  "y": 28}, {"x": 2,  "y": 55},
            {"x": 3,  "y": 43}, {"x": 4,  "y": 91},
            {"x": 5,  "y": 81}, {"x": 6,  "y": 53},
            {"x": 7,  "y": 19}, {"x": 8,  "y": 87},
            {"x": 9,  "y": 52}, {"x": 10, "y": 48},
            {"x": 11, "y": 24}, {"x": 12, "y": 49},
            {"x": 13, "y": 87}, {"x": 14, "y": 66},
            {"x": 15, "y": 17}, {"x": 16, "y": 27},
            {"x": 17, "y": 68}, {"x": 18, "y": 16},
            {"x": 19, "y": 49}, {"x": 20, "y": 15}
          ]
        }
      ],

"scales": [
        {
          "name": "xscale",
          "type": "band",
          "range": "width",
          "domain": {"data": "table", "field": "x"}
        },
        {
          "name": "yscale",
          "type": "linear",
          "range": "height",
          "domain": {"data": "table", "field": "y"},
          "nice": true
        }
      ],

"axes": [
        {"orient": "bottom", "scale": "xscale"},
        {"orient": "left", "scale": "yscale"}
      ],

"marks": [
        {
          "type": "rect",
          "from": {"data": "table"},
          "encode": {
            "enter": {
              "x": {"scale": "xscale", "field": "x", "offset":1},
              "width": {"scale": "xscale", "band": 1, "offset":-1},
              "y": {"scale": "yscale", "field": "y"},
              "y2": {"scale": "yscale", "value": 0}
            },
            "update": {
              "fill": [
                {"test": "datum === label", "value": "red"},
                {"value": "steelblue"}
              ]
            }
          }
        },
        {
          "type": "text",
          "encode": {
            "enter": {
              "align": {"value": "center"},
              "fill": {"value": "#333"}
            },
            "update": {
              "x": {"scale": "xscale", "expr": "label.x", "band": 0.5},
              "y": {"scale": "yscale", "expr": "label.y", "offset": -5},
              "text": {"expr": "label.y"},
              "fillOpacity": [
                {"test": "!label._id", "value": 0},
                {"value": 1}
              ]
            }
          }
        }
      ]
    };
    
    var runtime = vega.parse(spec); // may throw an Error if parsing fails
    var view = new vega.View(runtime)
      .logLevel(vega.Warn) // set view logging level
      .initialize(document.querySelector("#vis")) // set parent DOM element
      .renderer("svg") // set render type (defaults to canvas)
      .hover() // enable hover event processing
      .run(); // update and render the view
    
    // add a function to the dataflow to update pre with the hovered value
    function hoverfollower(evt, val) {
      d3.select("#pre-hovered").text(JSON.stringify(val));
    };
    // react to the label event stream
    view.addSignalListener("label", hoverfollower);
  </script>
</body>

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https://unpkg.com/vega@3.0.2/build/vega-core.min.js