Have you thought of normalizing the values? Say, per $1000?
I have. But normalizing to BTC volume seems to work better. It is simply the case that there is less variance in BTC volume than $volume, for obvious reasons.
Just from a lay persons perspective, a growth in Bitcoin's value is equal to deflation in the BitCoin Economy, and that fact alone will cause people to save (less BTC activity), however when there is a large increase in Dollar value that will cause a greater imbalance (more BTC Activity) To me it seems that it is both that are important to model.