If I understand you, you are relating this group's taxes to their income, which would be the average tax rate for the group.  This is also an exhibit on the Excel spreadsheet--but with some caveats.  First, the tax rate is computed based on the tax bill divided by "adjusted gross income".  Second adjusted income and the taxes include ordinary income, capital gains and dividend income--which I'd prefer to see seperately.  Third, if what you mean by "total tax bill" is intended to include all other taxes--state tax, sales tax, estate tax, gasoline tax, etc. etc. etc.,,,,it is not that.  Fourth, particularly as you move to the lower income groups, the concept of using adjusted gross income becomes problematic--because deductions such as child credits, dependents are available to this group, represent a larger % of income, and since they are deducted before the tax rate calculation, don't even show up in these figures--ie imply a higher tax rate for these grouups due to the impact of the deductions.

That being said, the exhibit shows tax rates have gone down for all groups shown on the exhibit.