Thanks
If you are talking about the trading volumes of individual exchanges, you can find that on bitcoin wisdom (https://bitcoinwisdom.com/).
You may add the volumes up, to have an approximation of total trading volume.
Thanks for the suggestion. I thought that would only go back a year but after some fiddling I see that's only BTC-e's chart (not that there's much to see before that).
You may also try the following three pages for data for BTC/USD, BTC/CNY, BTC/EUR.
http://www.cryptocoincharts.info/#jump-btc-usd
http://www.cryptocoincharts.info/#jump-btc-cny
http://www.cryptocoincharts.info/#jump-btc-eur
Just click "Charts" and then "All time", and you can get the daily data points easily.
But the site doesn't provide data for Huobi and OKCoin, but only BTCChina and Bter strangely.
Well they're thorough! Thank you again