“Mac gets back-to-school bump” is the headline of a brief report Saturday in Net Applications’ September survey. It notes that Apple’s (AAPL) share of desktop usage, as measured by visits to its clients’ websites from machines running OS X, rose 0.42 percentage points in September to a record (for Apple) of 6.45% worldwide and 13.7% in the U.S.

Microsoft (MSFT) still controls the desktop, of course, with a 92.44% share worldwide. But that’s down from 93.95% a year ago.

And although Net Applications attributes the Mac’s gain to parents and students gearing up for school, noting that “Mac usage generally rises in September,” the chart above makes clear that there’s something more going on. The bump in September 2010 is barely visible. The gains over the past two years are hard to miss.