Sales of Apple’s Mac line of personal computers slowed in April, leaving a question mark for analysts who have lofty expectations of Apple computers in the June quarter. Industry tracker NPD Group on Monday issued its estimates for Mac sales in the month of April. While the firm found that sales did grow compared to the same month last year, growth has slowed substantially. NPD says Apple shipped 9% more Mac computers in April 2011 than it did in the same month last year, but analysts are expecting shipments for the full quarter to jump 22% over 2010 to 4.2 million units. The decrease in growth is attributed to the fact that Apple refreshed its MacBook Pro line in the middle of April last year, but analysts obviously accounted for that when issuing estimates for Apple’s fiscal third quarter.