FLUX and Grafana graph - proper syntax / placement of "|> fill(usePrevious: true)"?

running influxdata OSS 1.8.2 and grafana 7.2.
i have measurement that when queried in regular influxdb and grafana with fill(previous) works, as expected.

converting to FLUX (which i need for soooooo many reasons) i get choppy stacked lines because some null points rather that fill(previous) creep in.

sample code snippet regardless of where i insert the fill line it does not appear to be functioning:
from(bucket: “app/autogen”)
|> range(start: v.timeRangeStart, stop: v.timeRangeStop)
|> filter(fn: ® => r._measurement == “M” and r.Ctag == “${C}” and (r.book == “A” or r.book == “C”) and(r._field == “relevant”))
|> fill(usePrevious: true)
|> window(every: 1m)
|> last()
|> fill(usePrevious: true)
|> pivot(rowKey:[“Ctag”], columnKey: [“book”], valueColumn: “_value”)
|> group(columns: [“Ctag”], mode:“by”)
|> fill(usePrevious: true)
|> map(fn: ® => ({
time: r._stop,
show_A: r.A,
show_C: r.C
}))