Vega Webinar - area chart from multivariable composite aggregation

webinar-demo2.hjson

{
  $schema: https://vega.github.io/schema/vega-lite/v2.json
  data: {
    url: {
      index: logstash-*
      %context%: true
      %timefield%: @timestamp
      body: {
        size: 0
        aggs: {
          table: {
            composite: {
              size: 10000
              sources: [
                {
                  time: {
                    date_histogram: {
                      field: @timestamp
                      interval: {%autointerval%: 500}
                    }
                  }
                }
                {
                  ext: {
                    terms: {field: "extension.raw"}
                  }
                }
              ]
            }
          }
        }
      }
    }
    format: {property: "aggregations.table.buckets"}
  }
  transform: [
    {calculate: "datum.key.time", as: "time"}
    {calculate: "datum.key.ext", as: "ext"}
  ]
  mark: area
  encoding: {
    x: {
      field: time
      type: temporal
      axis: {title: null}
    }
    y: {
      axis: {title: "Document count"}
      field: doc_count
      type: quantitative
    }
    color: {field: "ext", type: "nominal"}
  }
}