I am warned in advance that the "Visitor Locations" map on the right hand of the page is about to be reset to zero. The last time it happened I chalked it up to "server error -- everything reset to zero." But it seems it is not the case; they periodically start over so some blogs' charts don't become a bleeding mass of red.
It's annoying; a more accurate practice would be to keep a running tally of the last (100? 356? days) access, but then again that would require probably considerably more computation and unnecessary energy consumption for what is, really, a frill.
Oh well. There is an archive.