public class MyCustomChart : RadCartesianChart
{
    public event ValueChangedEventHandler SeriesUpdated;

    public delegate void ValueChangedEventHandler(object sender, EventArgs e);

    public void CallSeriesUpdate()
    {
        this.SeriesUpdated(this, EventArgs.Empty);
    }
}

Device.BeginInvokeOnMainThread(() =>
                    {
                        viewModel.AddPoint();

                        // call series update
                        chart.CallSeriesUpdate();
                    });

using Foundation;
using System;
using Telerik.XamarinForms.Chart;
using Telerik.XamarinForms.ChartRenderer.iOS;
using TelerikUI;
using TestXamarin.iOS;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(Telerik.XamarinForms.Chart.RadCartesianChart), typeof(CustomChartRenderer))]

namespace TestXamarin.iOS
{
    public class CustomChartRenderer : CartesianChartRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<RadCartesianChart> e)
        {
            base.OnElementChanged(e);
            this.DoAnnotationCustomizations();
            (this.Element as MyCustomChart).SeriesUpdated += CustomChartRenderer_SeriesUpdated;

        }

        // add annotation on series update
        private void CustomChartRenderer_SeriesUpdated(object sender, EventArgs e)
        {
            this.DoAnnotationCustomizations();
        }

        // add annotation
        private void DoAnnotationCustomizations()
        {
            // customization to the chart control
            // example with band annotation
            Foundation.NSDate nsDate = NSDate.Now;
            if (this.Control != null)
            {
                this.Control.AddAnnotation(new TKChartBandAnnotation(new TKRange(nsDate, nsDate.AddSeconds(200000)), this.Control.XAxis, new TKSolidFill(UIColor.Red), null));
            }
        }
    }
}