Today bitcoin has dropped by 20%, so I looked for alternative cryptocoins with better performance. But, hey! Litecoin, Iota, Ripple, Ethereum, all of that has the roughly the same graph along time in USD, EUR or even BRL. See, for example (from https://www.coingecko.com/en):
That does not make sense for me. "Oh, bitcoin is falling, I'll sell my litecoins" - I would buy altcoins.
Why altcoins (at least the ones listed above) are so related to bitcoin, instead of having independent variations?