I wonder why some fees appear to be below 0 sat/vbyte. Does this happen when two blocks are mined with a short time interval or is it a bug?
Probably a glitch in the graph when the fees go down
below zero, as far as I see this is indeed because the empty blocks that get mined as there were two close to those dates, the API just registers the zero sat fee reward.
That aside I'm happy it doesn't show prioritized transactions, like this one:
https://mempool.space/tx/d4c36c41b7e26305cdc3d248d13a62fc5dff8995b05a28231b3284d876aa002fwhich is really good as it will completely make a mess of that chart.
