Paintbrush Highlight

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "Select multiple points with the shift key.",
  "data": {"url": "data/cars.json"},
  "selection": {
    "paintbrush": {
      "type": "multi", "on": "mouseover",
      "nearest": true
    }
  },
  "mark": "point",
  "encoding": {
    "x": {"field": "Horsepower", "type": "quantitative"},
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "size": {
      "condition": {"selection": "paintbrush", "value": 300},
      "value": 50
    }
  }
}