You pay $2,000 in taxes on an income of $20,000, and a tax of $2,700 on an income of $30,000, then over this range of income the tax is

In this case the income tax would be regressive. Regressive tax is when the amount of tax charged decreases as the taxable income increases. For example in our case, at $20,000, $2,000 in taxes means a 10% tax rate. At $30,000, a tax of $2,700 means a 9% tax rate. As the taxable income goes up, the tax goes down.