If your wondering why the economy is so bad its because of this
Inflation adjusted percentage increase in mean after-tax household income in the United States between 1979 and 2005.
And the more the goverment grows, the larger the gap will be.
The rich can afford to move thier money around or buy whatever influence they need to avoid taxes and regulations (where do you think all these loopholes come from). It's always the middle class who end up suffering from the tax-the-rich class warfare.
As for the poor (bottom 5th), these number never include the aid they receive. If you included the Walfare, food stamps, wica, section 8 housing, etc, most these "poor" would end up closer to middle class income levels.
I also noticed the graph left out the middle/bottom 5th.
I guess the numbers for that group where a little to high and would have hurt your arguement.