I am interested here in the relative income growth between percentiles, so the inflation applies in the same way to all revenues (the value of the same £1 goes down, irrespective of how many £ you have). So as a %, you would have to shave the cumulative inflation between 1999-2013, but that's subtracting a flat line, it doesn't affect the shape of the % change curve.

The point is not so much that low incomes did 200%, it is that low income did much better than the rest of the distribution (relatively).