The market doesn't have to grow by 2 * PI anything. It's just a constant. Since the radius grows linearly, you can map any linearly growing series of data and you'll find it looks good in this presentation format.

You're right, just need to multiply by some scaling factor, as long as the growth stays linear.

So the way to resolve this would be to calculate the total users for each browser each year, and scale that entire ring outwards.