Take a look at SciChart for Xamarin - which provides rich, interactive high performance charts for iOS & Android.
Examples are hosted at Github: https://github.com/abtsoftware/scichart.xamarin.examples
A getting-started youtube video is here: https://www.youtube.com/watch?v=wYdab-0XHMA
Disclosure: I am the tech lead on the SciChart xamarin project
