As krays pointed out , it's an IHUB feature. And you didn't miss it for 4 years , it's a relatively new feature.
In adfvn's explanation of that information it says that a trade executed closer to the bid than the ask is recorded as a 'sell' and a trade executed closer to the ask than the bid is recorded as a 'buy'. Those right in the middle are recorded as '?'.
So , it's a rough estimate but it is kind of fun since it shows cumulative numbers in each column which update as the day progresses.