Flux: First, Last and the differnce per day

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To get the exact output format you showed above, you’d have to do something like this:

import "array"
import "date"

from(bucket: "ttn")
    |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
    |> filter(fn: (r) => r["_measurement"] == "waage-1")
    |> filter(fn: (r) => r["_field"] == "wight")
    |> window(every: 1d)
    |> reduce(
        identity:
            {   idx: 0,
                date: today(),
                "first time": today(),
                "first value": 0,
                "last time": today(),
                "last value": 0,
                difference: 0
            },
        fn: (r, accumulator) => {
            idx = accumulator.idx + 1
            date = date.truncate(t: r._time, unit: 1d)
            firstTime = if accumulator.idx == 0 then r._time else accumulator["first time"]
            firstValue = if accumulator.idx == 0 then r._value else accumulator["first value"]
            lastTime = r._time
            lastValue = r._value
            difference = lastValue - firstValue
            
            return {
                idx: idx,
                date: date,
                "first time": firstTime,
                "first value": firstValue,
                "last time": lastTime,
                "last value": lastValue,
                difference: difference,
            }
    })
    |> drop(columns: ["idx", "_start", "_stop"])

But if you’re just trying to show the difference between the first and last value of each day, there’s a simpler way.