Already resolved itself, the Nodes counter had the same dip so probably just servers not responding.

Where can I see this chart?

This is a bug. The crawler don't finished to get the data for the last time unit and the result is this drop. What some hours and it will be ok.

Where do can we see these graphs?

Just guessing, but maybe this has something to do with instances overload

Looks like a server fault. The Mastodon user count bot sometimes goes buck wild too.