Sir, when you check whole graphic you can understand easly, you shouldn't buy btc when it became 1500$
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Why not? Perhaps it will be $1000000 a few months from now.
It is impossible to predict if the exchange rate will increase or decrease. Therefore, you should only exchange for bitcoin money that you can afford to lose. You can't know today if bitcoin will be $5000 or $5 next week.
The "whole graphic" only makes sense because you are looking at the past, at things that have already happened, so you know how it turns out. Unfortunately, when you choose to acquire some bitcoins, you can only look at the present. You can't know how it will turn out, so the graphic isn't useful.