what dates are these?

[axd1967]

Various graphs show several vertical white lines. What dates do these represent?

[10110111]

They look like simple grid lines to me, with some of them missing for some reason. In fact, in your screenshots secondary grid lines are also missing completely. Here's how it looks for me:

Is there some unusual setting, CSS or anything you're using?

[axd1967]

Not using any of those; I noticed the difference because I use these graphs often.

Indeed it looks like a display artifact; the exported graph looks normal

I'm using AppImage 26.2.qt5

AppImage 26.1.qt5 has the same issue
AppImage 25.2.qt6 does not have the issue
AppImage 26.2.qt6 also has the issue