Signal "flatlines" when phone is plugged in to charge.

It appears that when I charge my phone at night, the signal flatlines to its lowest reading over all the testing period. I leave the phone on so it continues to record, so it's not because the phone is powered down.

Anyone else's phone do this, too? Maybe the electric current causes some type of geomagnetic interference (not surprisingly), but I would have expected it to flatline at a higher, rather than baseline number. Of course, I'm not a geophysicist, so this was a layman's expectation, rather than based on some deep scientific insights : -D