There were a couple of similar issue back in October 2020:
What happened here? jochen-hoenicke.de.
Confusing a website's mempool stats with the Bitcoin networkBoth point to a node restart being the likely reason behind the glitch, although the one shown on the image here is weird, as the datapoints show a descending regular line between somewhere before 00:00 and after 04:00 which is different to the behaviour on those other referenced cases.
Can’t see the effect displayed on the first image on the live site now.
Edit:
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True. The node seems to have been unreachable for a longer period of time than on prior occasions, thus the descending regular line over different data points that connect last read with restart of readings (presumably from the near 0 baseline).