This graph could just as easily be explained as people with high incomes (Mac users generally do) are more insulated from economic downturn than the people buying $300 laptops. Reading it how you want doesn't make it true.

Even according to the graph, 15x means that one in 16 machines sold is a Mac. Math tells us that's 6.25% market share. That number hasn't appreciably changed in real terms in the last decade. Yes, moving from 4% to 6% is a 50% "growth", if you want to define growth that way. Normally, that's 2% growth.

However, the inability of Mac to grab any significant market share from the overall PC market should be a concern, considering the relative popularity of their other devices.