<telerikChart:RadCartesianChart x:Name="chart">
    <telerikChart:RadCartesianChart.BindingContext>
        <local:ViewModel />
    </telerikChart:RadCartesianChart.BindingContext>
    <telerikChart:RadCartesianChart.HorizontalAxis>
        <telerikChart:DateTimeContinuousAxis LabelFitMode="Rotate" 
                                                x:Name="horizontal"/>
    </telerikChart:RadCartesianChart.HorizontalAxis>
    <telerikChart:RadCartesianChart.VerticalAxis>
        <telerikChart:NumericalAxis />
    </telerikChart:RadCartesianChart.VerticalAxis>
    <telerikChart:RadCartesianChart.Series>            
        <telerikChart:SplineSeries ValueBinding="Value"
                                    CategoryBinding="Date"
                                    ItemsSource="{Binding Data}"
                                    ShowLabels="True"
                                    />
    </telerikChart:RadCartesianChart.Series>
    <telerikChart:RadCartesianChart.Annotations>
        <telerikChart:CartesianGridLineAnnotation Stroke="#0E72F6" 
                                                StrokeThickness="2"
                                                Value="{Binding Threshold}"
                                                Axis="{x:Reference horizontal}">
            <telerikChart:CartesianGridLineAnnotation.DashArray>
                <x:Array Type="{x:Type x:Double}">
                    <x:Double>4.0</x:Double>
                    <x:Double>2.0</x:Double>
                </x:Array>
            </telerikChart:CartesianGridLineAnnotation.DashArray>
        </telerikChart:CartesianGridLineAnnotation>
    </telerikChart:RadCartesianChart.Annotations>
</telerikChart:RadCartesianChart>  

private void Button_Clicked(object sender, System.EventArgs e)
{
       this.chart.Zoom = new Size();
}

private void OnResetZoomClicked(object sender, EventArgs e)
{
	this.chart.Zoom = new Size(1, 1); // This line is added as a workaround to reset the Zoom level
	this.chart.Zoom = new Size();
}