Fluent Style Pie Charts and Bar charts on .NET UWP C#
Two main options that I know of:
Microcharts
Microcharts is a pretty simple library that has some Fluent qualities by default but lacks customization options. It doesn't look like it's getting any further support from it's maintainer.
You can find it here.
Telerik
Telerik has an open-source charting control for UWP that's much more flexible but takes some more work to get a nice aesthetic polish. I've seen some pretty well-done visuals made with this library, but it's also receiving minimal updates (all focus seems to be on WinUI right now).
You can find it here.