Each day's volume divided by the median volume of the days
days before it: 2 is twice the usual volume of the time, 0.5 half of it.
The median keeps a single busy day from raising the bar, and the ratio
follows the coin's volume as it grows over the years. Days without volume
(0 or missing, as 'CoinMarketCap' has for the earliest days of some coins)
are left out, and a day needs volume on two thirds of the days before it.