Thank you for using Syncfusion product.
We have analysed your sample code and you have to load the Syncfusion.SfChart.XForms.iOS and Syncfusion.SfChart.XForms.WinPhone assemblies manually by creating instance for SfChartRenderer in WP and iOS as Xamarin is not loading it automatically.
In WinPhone Project
Create an instance of SfChartRenderer in MainPage constructor,
In iOS Project
Create an instance of SfChartRenderer in FinishedLaunching overridden method of AppDelegate class,
public override bool FinishedLaunching(UIApplication app, NSDictionary options)
new SfChartRenderer ();
For your reference, we have also prepared the sample and please find the sample in the below location,
For more details please refer the below document link:
Please let us know if you need any further assistance.