Chart: Apple’s first weak back-to-school Mac quarter since 2002 by Philip Elmer-DeWitt @FortuneMagazine October 10, 2013, 11:38 AM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons Data: Gartner. Chart: MacRumors. FORTUNE — An Apple AAPL summer tradition — strong back-to-school sales of the Mac — finally got broken by the same disruption that has been draining growth out of the entire PC industry: The booming tablet market that Apple created when it launched the iPad in 2010. Gartner and IDC released their third quarter PC shipment reports Wednesday, and while almost none of their numbers agree, they both report Mac sales down year over year for the first time in a back-to-school quarter since the fall of 2002. Below: Gartner’s and IDC’s U.S. reports. See the links below for their worldwide numbers. Thanks to MacRumors for the line chart. Links: Gartner press release. IDC press release.