That's very interesting! I think Adam4eve has something similar by comparing trading volume between days:
dev.adam4eve.eu/margin_finder.php?cpage=1
So if you check A4E tracker: Volume, "BuyfSell" means how much volume is being bought from existing orders in a day, and "Sold2Buy" means how much is being sold to the buy orders. It has a Buy/Sold ratio so if the ratio is greater than 1, it means more items were bought by people from sell orders, than sold to buy orders. Which is generally good if the ratio not being too high.
I think it snapshots from EVE API hourly to get the estimated volume, not 100% precise, but the trend is good enough to tell and matches the trade graph in game