Hello iampil,
If the signal strength over time line is broken, that just means we missed beacons from that access point and couldn’t plot the line.
Access points announce their existence by “beaconing”, or sending out a frame saying that they are there. They still have to take turns talking with everything else on the channel, so sometimes, they might miss their target beacon interval.
There could also be non-Wi-Fi interference, or the access point could just be too far away. It could also be band-steering mechanism from the AP or adapter to encourage roaming to the 5 GHz band.
I hope this helps!