There really is no such thing as a "fair rate" when it comes to cryptocurrency trading. People buy and sell at prices based on a lot of different factors. For example, I bought thousands of Mintcoins when people were selling them at 40 satoshis each. Now that the market is valuing them at around 25 satoshis apiece, I could very well be inclined to say that I bought too early. However, I am not. I bought the coins because I believed that the price would increase sometime in the future. I could have waited to buy until the price was lower, but no one can accurately predict market trends. It was also to my advantage that I bought early because time is a very crucial factor when it comes to minting, and I bought those coins with the sole intent of eventually minting them. So did I buy at a fair rate? From my perspective, yes. If I could have bought at a lower price, then I would have, but the question of fairness would once again come about, only this time on the side of the seller. I guess one could say that "fair" is a measure of how much you value a coin relative to how much the market values it. Those prices you see there are measurements of how the market currently values Mintcoins; the question of fairness essentially translates to "Given all that you know about Mintcoins, are you willing to spend that much to buy them or not?"
EDIT:
It seems like you had a question within your question.
That chart is just a representation of the trades that occurred within the days listed on it. The high spikes show trades where buyers bought Mintcoins at a price much higher than the average market price, while the dips show trades where buyers bought at a lower price. Hover over the bars to see more information.