Brushing/linking in vega (not vega-lite)

Question

I'm trying to brush/link two plots in vega, more specifically a node-link diagram and a couple of scatterplots. Based on how dragging works with signals in the node-link diagram I did get quite far, but not far enough...

For the sake of simplicity, I'll provide a little test code here using just two scatterplots:

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "padding": 0,
  "autosize": "none",
  "width": 800,
  "height": 400,

  "signals": [
    { "description": "Any datapoint is activated",
      "name": "datapoint_is_activated", "value": false,
      "on": [
        {
          "events": "symbol:mouseover",
          "update": "true"
        },
        {
          "events": "symbol:mouseout",
          "update": "false"
        }
      ]
    },
    { "description": "Active datapoint",
      "name": "activated_datapoint", "value": null,
      "on": [
        {
          "events": "symbol:mouseover",
          "update": "item()"
        },
        {
          "events": "symbol:mouseout",
          "update": "null"
        }
      ]
    }
  ],

  "data": [
    {
      "name": "table",
      "values": [
        {"name": "point A", "a": 1, "b": 2, "c": 3},
        {"name": "point B", "a": 4, "b": 5, "c": 6},
        {"name": "point C", "a": 9, "b": 8, "c": 7}
      ]
    }
  ],

  "scales": [
    { "name": "xscale",
      "type": "linear",
      "domain": [0,10],
      "range": [0,200]
    },
    { "name": "yscale",
      "type": "linear",
      "domain": [0,10],
      "range": [0,200]
    }
  ],

  "layout": {"padding": 20},

  "marks": [
    { "name": "plot1",
      "type": "group",
      "axes": [
        {"orient": "bottom", "scale": "xscale"},
        {"orient": "right", "scale": "yscale"}
      ],
      "marks": [
        {
          "type": "symbol",

          "from": {"data": "table"},

          "encode": {
            "enter": {
              "x": {"field": "a", "scale": "xscale"},
              "y": {"field": "b", "scale": "yscale"},
              "tooltip": {"field": "name"}
            },
            "update": {
              "size": {"value": 100},
              "fill": {"value": "grey"}
            }
          }
        }
      ]
    },
    { "name": "plot2",
      "type": "group",
      "axes": [
        {"orient": "bottom", "scale": "xscale"},
        {"orient": "right", "scale": "yscale"}
      ],
      "marks": [
        {
          "type": "symbol",

          "from": {"data": "table"},

          "on": [
            {
              "trigger": "datapoint_is_activated",
              "modify": "activated_datapoint",
              "values": "{fill: \"red\"}"
            },
            {
              "trigger": "!datapoint_is_activated",
              "modify": "activated_datapoint",
              "values": "{fill: \"grey\"}"
            }
          ],

          "encode": {
            "enter": {
              "x": {"field": "a", "scale": "xscale"},
              "y": {"field": "c", "scale": "yscale"},
              "size": {"value": 100},
              "tooltip": {"field": "name"}
            },
            "update": {
              "fill": {"value": "grey"}
            }
          }
        }
      ]
    }
  ]
}

The resulting image looks like this:

The idea is that hovering over a datapoint in the left plot will highlight the corresponding datapoint in the right plot. I know this is straightforward in vega-lite, but that does not (yet) support node-link diagrams. Hence: vega.

My approach in the code is to:

create a signal in the outer scope that tracks (a) if there is a point activated, and (b) which point this is
in plot 2 to have a trigger that takes the activated datapoint and changes its colour to 'red'.

I have an inkling feeling that it has to do with my definition of the modify and values part, but I can't figure it out.

Any help very much appreciated!

Thank you, jan.

Brushing/linking in vega (not vega-lite)

Answers (1)

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