url: {
  // "type" defaults to "elasticsearch" otherwise
  type: emsfile
  // Name of the file, exactly as in the Region map visualization
  name: World Countries
}
// The result is a geojson file, get its features to use
// this data source with the "shape" marks
// https://vega.github.io/vega/docs/marks/shape/
format: {property: "features"}