Yes, altcoins prices are based on bitcoin price but not all. Try to look at the market prices when bitcoin is down, most altcoins are down also but as what I have said not all. There are still altcoin that is continuously rising even if bitcoin price is down.
I don't think all altcoins depend on the price of bitcoin. well, I only know that what really affects the price of altcoin is ETH. well, you can see at this time that when the ETH price is down, a lot of altcoin prices are also decreasing.