I'm not sure I understand your point. The data is normalized to population, it's in quintiles and percentiles. It doesn't matter how many millionaires there are, the top 1% is still the top 1%. What is your contention exactly? That there is some sort of ultra-rich top 0.1% or 0.01% who are shirking their tax liability and leaving the rest of the top 1% to carry it? Or that the top 1% is not large enough of a grouping of high-income folks and that somehow they're paying too much and the top 5% or so should be paying more?
Regardless of the tax rate the actual tax revenues from the top 20, 10, and 1% has increased substantially in the last several decades, to the degree such that only 40% of households are contributing fully 85% of federal tax revenues today.
You're right about the data being normalized, so it doesn't matter how many millionaires there are, and the top 1% is still the top 1%. However, that top 1% can be a richer 1%. Take this from the article:
In the late 1970s, the top 1 percent took home 9 percent of total national income. Now the top 1 percent’s take is more than 20 percent.
The chart you referenced shows tax revenues on the top 1% roughly doubled for the time period from 1979 to 2007, but over that approximate time period so did the total percent of the national income going to the top 1%! That would appear to explain much, if not all, of the increase right there. One wouldn't be able to take home double the income yet pay the same in taxes without serious loopholes.
Regardless of the tax rate the actual tax revenues from the top 20, 10, and 1% has increased substantially in the last several decades, to the degree such that only 40% of households are contributing fully 85% of federal tax revenues today.