The progression is really non-linear and strong in the low part, weak in the hight. I made a graph to show you what I mean:
This graph shows a flat income tax (10%) with 3000 prebate. Y is the % of the tax you pay, X is your income. As you see, you have a huge progression under 200000 and than the curve get's almost flat.
But this is still an income tax. If you now go to a VAT like tax, you would have a different curve because not everyone spends as much money on goods and services.