houarago.js

[{
    $match: {
      reading: {  $gt: 1000, $lt: 40000 }
    }
  }, {
    $setWindowFields: {
      partitionBy: "$type",
      sortBy: { date: 1 },
      output: {
        hourago: {
          $min: "$reading",
          window: {
            range: [-3600, 0],
            unit: "second"
          }
        }
      }
    }
  },
  {
    $set: {
      meterdelta: { $subtract: ["$reading", "$hourago"] }
    }
  }
]