I guess it's true that those straight lines appear in your all time history. I tried looking at your archive for an explanation but it appears you had plenty of games during the period in 2021 of October 2021 to January 2022 apparently shown by the second instance of the straight line.
I am guessing that the problem may be caused by the extraordinarily large number of games you have played.
When I open the All Time tab on my Blitz performance page, I see two long flat lines in 2020 and 2021, which weren't there a few days ago. I never had a period of not playing chess on the site for more than a couple of days. My overall rating also dropped because of this. Anybody else experiencing this bug? I couldn't find another post on this topic.