If you wanted to see the effects of the PoS switch (3 months ago), you'd have to choose a shorter timescale, like 6 months. It's been going down slightly, but not drastically, and only in proportion to the decrease in daily transaction count (as simpsond mentioned).

PoS hasn’t had much impact. The leveling out there is more like related to fewer transactions. Proto dank sharding, which may ship in 2023, should have a bigger impact on gas fees.