After struggling with iMac sales last holiday season, Apple finally sees improvement in U.S. Mac sales.  In January, the sales of all-in-one iMac grew 31 percent.  Why so sudden growth? Apple didn’t have enough redesigned iMac in stock.  With improved supply, Apple is able to sell more new iMac line.  The current data is an important leg for Apple.  That signals Apple’s success with iMac line.