[tomkarlo]

If the ratio of taxes to income isn't rising with income or at least keeping constant (which is what I think you're referring to as "linear"), your marginal tax rate is declining as income rises. I think you maybe mean it's staying constant with consumption, but that's kind of the problem - people with higher incomes tend to consume a smaller portion of their overall income, so taxes based on consumption are usually regressive.

That's pretty much the definition of regressive when it comes to taxes - people making more are being taxed less, proportionally.

http://en.wikipedia.org/wiki/Regressive_tax