I would prefer to have the heatmap in BTC rather than USD
Mind if I ask why?
In the long term, the price of a BTC is likely to have an overall upwards drift. Therefore, if the heatmap is in USD, data from the early years will probably become invisible.
On the other hand, there are already 7.6 million BTC, and the total quantity won't rise above 21 million BTC, so in the long run the BTC volumes will make for consistent charts. (I admit it's not quite so simple, because active traders are exchanging the same coins over and over again.)
Another reason to prefer BTC to USD is that the choice of USD is somewhat arbitrary. Over time, BTC exchange might shift towards other currencies such as sterling, the euro, or renminbi. But BTC will always be a useful and relevant figure, no matter what currency the BTC are being exchanged against, or by how much the fiat currency has inflated.