Oh Blah Dee Blah Dah

Apple's iTunes: Rapid Sales Acceleration

Your chart is poorly drawn. The "y" axis should show each billion song milestone, and the "x" axis should show the date at which each billion was reached.

Then, the chart would show a steepening, upward sloping curve with each billion milestone CLOSER to the previous milestone.

This would demonstrate that each billion milestone is occurring EARLIER than the previous milestone.

In other words, Apple has been consistently selling the next billion songs FASTER than the previous billion songs.