Trellis Histograms

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/cars.json"},
  "mark": "bar",
  "encoding": {
    "x": {
      "bin": {"maxbins": 15},
      "field": "Horsepower",
      "type": "quantitative"
    },
    "y": {
      "aggregate": "count",
      "type": "quantitative"
    },
    "row": {"field": "Origin", "type": "nominal"}
  }
}