What does this chart means? A decreasing value means that you get less easily USD while mining BTC, with constant hardware. It let you see the potential correlation between BTC price in USD and difficulty variations (fully correlated = stable value).

The forumula is: MiningProfit(Date) = ExchangeRate(Date,BTC-USD) * AvgDifficulty(Date,USD) / AvgDifficulty(Date,BTC)