If this graph were true wouldn't we be seeing attacks all the time? If coins could be exploited for just hundreds of dollars they'd be rendered useless pretty quickly. This can't possibly be accurate.
1) If you can pull off an attack for a few hundred bucks, it's probably on a coin that will only net you a few hundred bucks. Remember, these coins are thinly-traded altcoins, many of which only do a few hundred bucks worth of volume on exchanges in the first place.
2) Once you pull off your attack and people realize what happened, the "free" coins you get back will be worth shit because nobody will want that coin.
It's not as much of a free lunch as it might seem at first blush. I most certainly possess the domain knowledge and skill to pull off a 51% attack, as I have contributed code to Bitcoin and many other cryptocurrencies, written open source mining software, etc. But I would need several million dollars in starting capital to make the kind of money that would make me even consider it. That time would be better spent hacking away on a project with the potential to make me sustainable income, or a client project where I'm paid by the hour.
On point 2 I thought the entire point was to double spend - so you have 1 einsteinium or whatever, you put it in the exchange, you use it to buy bitcoin. You re-spend it at another exchange buying bitcoin. You got twice as much bitcoin as you paid for. As long as you're out before it becomes obvious what you've done you make profit.
What is quite ironical, because if any of them just become a useful currency, that means it would get instantly attacked and worthless.