I created such a graph but it's been not updated since March because I couldn't find the trade data. http://bitcoin.atspace.com/income.html
I'll update it as soon as I get the data.
In long term, the mining income decreased in USD, i.e. difficulty increased faster than BTC/USD. Each jump in profitability resulted in miners extending their mining operation and difficulty going up.
edit: Updated thanks to bitcoincharts data. With the recent BTC increase, the mining income rose but it's still far below the levels of the small stabilization at the beginning of the year.