I haven't updated Xbox pricing data, but here's a different chart showing inflation-adjusted PlayStation price trajectories over their first six years:
The PS4 is as of September 2018 more expensive than any other PlayStation system was by their respective fifth September. The PS1 was the least expensive overall through most of its life, the PS3 was the most expensive through most of its life, and the PS4 started off on par with the PS2 and has spent most of the past four years between the PS2 and PS3. The PS4 has also received the shallowest and fewest permanent price cuts. Historical precedent suggests that if we do get another permanent price cut for the PS4 Slim, it will be down to $250 at most. While some have speculated a $200 price point, historical data makes me skeptical that we'll see a price cut that deep this late in the system's life. I think $250 for the Slim and $350 for the Pro are the likely final price points for the PS4 before the PS5 debuts, assuming either one gets a permanent price drop.
I've noticed that over the years, the price on software drops faster and faster, especially used games at Gamestop. However, new games drop to $40 then $20 just after a few months. They may have to keep hardware prices up longer to compensate the lower profit on software. I was surprised how long it took the PS3 price to come down, and I am even more surprised how hesitant they are to drop the PS4 price.