This graph only says a tiny bit about one small market niche: College students in the United States (and, as you might know, that's only a small part of the world). Traditionally, Apple has been catering to that American niche market very well, especially in California. But it never helped them gain a significant market share.
In the business sector - where the real mass volumes are sold - Apple still is not a player. Apple neither has a product line for this massive market nor the necessary service infrastructure or support options. When you have 3,000 or more computers to take care of, you don't walk to an Apple Store or wait for an appointment with a "genius" if you need service. You want the four-hours-same-business-day option that Dell or HP give you. And you need a business partner that sells you and supports real server equipment, including backup devices and storage systems.
Apple also only has the required software compatibility with the business world when you put an operating system on their hardware that Apple still does not officially support: Microsoft Windows. (Or, worse, Linux.)
By the way, this graph - naturally - says nothing about the used operating system on those notebooks: How many operating systems are installed on them and which one is used more often or which is more important for the user from a professional/work-/study-related perspective.
Although most Mac users seem to believe that the iLife-suite is all that you will ever need, in the real world where you have to make a living somehow, iLife is just a collection of almost useless toy software. And Microsoft Office isn't the (only) answer, either - it's just one brick in the wall. But as much as Dell and HP have the hardware to succeed in the enterprise sector, Microsoft has all the necessary software products for it - and their server-side stuff is really good and extremely successful.
Apple, on the other side, almost exclusively targets the consumer market, and there only the upper middle and higher price segments.
I still don't see "the writing on the wall", as one poster here wrote.
Apple could significantly increase their market share and importance if they opened OS X to OEMs -- OEMs who have experience in those markets where Apple cannot compete. It's better to make a buck on software license sales then not make a buck at all. Ask Microsoft, that's how they became so big.