Suddenly Coinbase is bigger than Finex and Bitstamp

I remember some weeks ago they had really low volume
No. Since they started charging fees they have lower volume than Bitstamp. Coinbase ask side (and bid if we take out that wall) is still very thin. Don't know why would anyone wire 1mil$ to the exchange with such low liquidity.
Look at Chart Buddy's charts.
Unless it is a bug or the graphs are not in the same scale, something is wrong