{
  "$schema": "urn:prism:schema:v1:spec",
  "title": "Actual vs Benchmark (two-layer composition)",
  "datasets": {
    "actual": {"source": "testdata/cohorts/actual.pulse"},
    "bench":  {"source": "testdata/cohorts/benchmark.pulse"}
  },
  "resolve": {"scale": {"y": "shared"}},
  "layer": [
    {
      "$schema": "urn:prism:schema:v1:spec",
      "data": {"name": "actual"},
      "transform": [
        {"data": "actual", "groupby": ["brand_id"],
         "aggregate": [{"op": "mean", "field": "score", "as": "actual_mean"}]}
      ],
      "mark": "bar",
      "encoding": {
        "x": {"field": "brand_id",    "type": "nominal"},
        "y": {"field": "actual_mean", "type": "quantitative"}
      }
    },
    {
      "$schema": "urn:prism:schema:v1:spec",
      "data": {"name": "bench"},
      "transform": [
        {"data": "bench", "groupby": ["brand_id"],
         "aggregate": [{"op": "mean", "field": "median_score", "as": "bench_mean"}]}
      ],
      "mark": {"type": "rule", "stroke": "#999999", "stroke_dash": [4, 2]},
      "encoding": {
        "x": {"field": "brand_id",   "type": "nominal"},
        "y": {"field": "bench_mean", "type": "quantitative"}
      }
    }
  ]
}