Capital gains ARE income taxes.  These are income tax return data statistics, which include capital gains taxes.  They use a different worksheet for the calculations (Schedule D), but they're still part of your income tax return (Form 1040).  

Again, you're incorrect that higher incomes pay lower rates (on average).  It's just the opposite - they pay HIGHER rates than all the brackets below them (on average).  

If you pay 30% of your total income in federal income tax, you're likely doing something wrong.  The average effective rate for a middle class taxpayer is usually in the 5%-15% range.  Again, we're talking your federal income tax return. Not state income taxes, not FICA taxes, not property or sales taxes, etc. Just federal income taxes files on your 1040. Take a look. It's extremely unlikely that you pay anywhere close to 30%.  I paid 10% last year (tax amount / total income).