Difficulty definitely affects how difficult it is to get Bitcoin through mining but the thing you have to bear in mind is that the laws of supply and demand always apply here, if people are willing to purchase it then it doesn't matter if 'experts' think it has no value it will go up regardless if they're happy with paying that price in paper money. When it comes to altcoins I have noticed a definite trend though more than in Bitcoin of difficulty affecting price but when a coin gets popular then obviously people will buy into it regardless of the difficulty and of course the hashing power that you have on the network is directly tied to how much or how fast the difficulty rises depending on the rules the altcoin follows.
Yes. Since difficulty directly affects the possible amount of bitcoin supply by definition, I thought it might be reasonable to indicate bitcoin's current value looking at the provided graph. On the other hand, there are numerous factors that affect the demand. It might be harder to indicate whether bitcoin is overvalued just by trying to assume whether the demand will increase.
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