Spent Output Price Distribution (SOPD) shows at which prices UTXOs were spent on a given day (i.e. each bar shows the amount of Bitcoin volume that was moved within that specified price bucket). The price specified on the x-axis refers to the lower bound of that bucket.
There are two variants of this metric based on x-axis partitioning:
ATH-partitioned (price buckets are defined by dividing the range between 0 and the current ATH in 100 equally-spaced partitions)
% partitioned (price buckets are defined by taking the day's closing price and creating 50 equally-spaced bucket each above and below the closing price in steps of +/- 2%)
SOPD price buckets are calculated by adding the value of all spent outputs where the price when the spent outputs were created falls into that given bucket.
Glassnode - Twitter Thread