influxdb - InfluxDB Flux 当前值减去 90 天前的值

Question

日期	价值
2020-04-06	100
2020-04-05	90
------	-----
2019-09-06	60
2019-09-05	50

如何使用通量低于 rsult：

日期	减去 val
2020-04-06	40
2020-04-05	40
------	-----
2019-09-06	...
2019-09-05	...

现在我普通使用map，像这样

from(bucket:"example-bucket")
  |> filter(fn: (r) =>
    r._measurement == "cpu" and
    r._field == "usage_system" and
    r.cpu == "cpu-total"
  )
  |> range(start:-12h)
  |> map(fn: (r) => ({ r with _value: r._value - r.<Get 90 days value>}))