Scatter Plot with Text Marks

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {"url": "data/cars.json"},
  "transform": [{
    "calculate": "datum.Origin[0]",
    "as": "OriginInitial"
  }],
  "mark": "text",
  "encoding": {
    "x": {"field": "Horsepower", "type": "quantitative"},
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "color": {"field": "Origin", "type": "nominal"},
    "text": {"field": "OriginInitial", "type": "nominal"}
  }
}