Im fairly certain they're using it in Battery Health and Screen Time.
I've had a chance to use Swift Charts in some personal apps and am really impressed by it. It's so nice to have a native charting library available out of the box.
As with all Apple APIs (especially newer ones) watch the WWDC sessions and read the Apple documentation first instead of trying to sling it. It makes it way more intuitive.
Recently, a post about the limits of box plots went through HN. It praised violin plots as better alternatives[0]. Are those supported in Swift Charts?
Disclaimer: I worked on Swift Charts at Apple and I'm very excited to see it used!