Rectangular Brush

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "Drag out a rectangular brush to highlight points.",
  "data": {"url": "data/cars.json"},
  "selection": {
    "brush": {
      "type": "interval"
    }
  },
  "mark": "point",
  "encoding": {
    "x": {"field": "Horsepower", "type": "quantitative"},
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "color": {
      "condition": {"selection": "brush", "field": "Cylinders", "type": "ordinal"},
      "value": "grey"
    }
  }
}