Every investor has a certain amount invested in the bankroll. All investors together build the bankroll (bankroll = sum of all investments).
The current bankroll is 1588BTC, of which eg Reve364 owns 54.95BTC (-> 3.46% of the bankroll).
Out of all profit the casino makes, they will receive 3.46% (after fees and all).

Because total profit is simply the sum of all profit the casino made until the present day (meaning historic numbers from June 2015 until the present day, historic numbers that do not influence current investments).
Invested amounts are not purely profit though, they combine between a base amount that the investor deposited and profit that has since been added to that base investment.
Further, new investors coming into the bankroll or old investors pulling out with profit are not being measured into that chart, as it's a simple global profit chart (unlike the bankroll size chart, which takes those things in).