If you can write a bit of Javascript you could build a custom chart to do that. You could also adjust your zones so the time in zones stuff answers your question.
You could also create a computed activity field to show how much time was under 80%.
Here is a script that will calculate what percentage of the time for the ride was spent at less than 80% of FTP:
{
let watts = streams.get("fixed_watts").data
let time = streams.get("time").data
let ftp = activity.icu_ftp
let secsUnder80 = 0, tot = 0
for (let i = 0; i < watts.length; i++) {
let w = watts[i]
let secs = time[i] - (i > 0 ? time[i - 1] : 0)
if (secs >= 30) continue // coffee stop
if (w < ftp * 0.8) secsUnder80 += secs
tot += secs
}
tot > 0 ? (secsUnder80 / tot) * 100 : null
}