Well the thing is that it would be easy that way to manipulate transaction fee charts in Ripple (since fees there are several orders of magnitude smaller than in PoW coins) and other similar coins and fees there do not necessarily represent the amount of "investedness" into the system. Transaction volume might be another indicator that looks nice at first, still it again can be easily gamed (I can afford to replicate every single transaction ever in all of Bitcoin into Ripple for XRP worth less than 20 USD in fees) in coins that have smaller fees.