Is there any website, which allows to choose few exchanges like Poloniex and Cryptopia and see all those exchanges Bitcoin price chart in one chart or show multiple small charts which can fit to one browser window so each can be seen same time?
TradingView allows up to eight panes for different charts in one screen, so you can watch up to eight assets or markets at once. I find this useful for measuring relative strength among the West vs. Korea vs. Japan.
They support the major BTC exchanges and some altcoin exchanges like Poloniex and Bittrex. But they don't support smaller exchanges like Cryptopia. You need a standard Pro account for multi-pane charts, which costs somewhere in the ballpark of $10-15 a month, depending what package you buy.