Billionaire Tax Calculator

This is really strange. The ProPublica article says the "IRS records show that the wealthiest can — perfectly legally — pay income taxes that are only a tiny fraction of the hundreds of millions, if not billions, their fortunes grow each year." I had to make a double-take on this. The authors are implying that "wealth growth" and "income" are the same and based on this, they calculate the percentage of income tax they think wealthy people should have paid on their wealth growth!? How is that logical? ?makers