Well exchange show you the value of the coins ,mostly converted into bitcoins,but they dont show you the gain or lost ,only shows bitcoin and total dollar value .Poloniex shows those way the others i dont know but well you can easy make some document and see the volatility of any coin.
Yes, this is true. You can see complete value of all of your coins in many exchanges, poloniex included but even though this is handy, I don't advise you to keep any amount of coins on the exchanges. This is way to risky. Take care of your coins by yourself!