...that shows the share of federal revenue from different tax sources. (Look at that green line.)
From the Wikipedia page, "Federal Insurance Contributions Act tax" (my emphasis):
The Center on Budget and Policy Priorities states that three-quarters of taxpayers pay more in payroll taxes than they do in income taxes. The FICA tax is considered a regressive tax on income with no standard deduction
or personal exemption deduction. It was imposed for the years 2009,
2010 and 2011 on only the first $106,800 of gross wages and increased to
$110,100 in 2012 and $113,700 in 2013. The tax is not imposed on
investment income such as interest and dividends.
Rather than all this talk of means-testing Social Security and Medicare, why not just lift that cap a little?