Charts in Xamarin.Form

Hi guys,

I want to show some bar charts and pie charts in a Xamarin.Form App, which tool is good and easy to use?(free ones)
If I want to do this, must I do some adjustments in the Xamarin.IOS project? Or just add do it in the shared project?
I mean I did everything just in the shared project. I still don't know how to add classes in the Xamarin.ios project. Is there anything helpful?

Thank you!