Im not sure how these stats are collected, I assume that they query each server for its to make the chart, rather than query every server every day and copy the results.

If they’re really copying the results, then you’re absolutely correct that temporary instances outrages would cause those correlated downward blips, but I’m surprised to hear you wouldn’t just be able to query servers to get this data on demand.
But then again if a server went permanently offline you’d lose that data forever. Hmmm