Why I can't see VerticalLineAnnotation Label?

Hello,
I have this

private readonly string CHART_MARKER_TEXT = "Marker";
private readonly string CHART_MARKER_STYLE = "VerticalLineAnnotationStyle"; // defined in Chart.xaml

    AddVerticalLineAnnotation(new AnnotationRequest() { XValue = 3, LabelText = CHART_MARKER_TEXT, AnnotationStyle = CHART_MARKER_STYLE });

private void AddVerticalLineAnnotation(AnnotationRequest request)
{
    Annotations.Add(new VerticalLineAnnotationViewModel
    {
        X1 = request.XValue,
        LabelValue = !string.IsNullOrEmpty(request.LabelText) ? request.LabelText : null,
        VerticalAlignment = VerticalAlignment.Stretch,
        LabelPlacement = LabelPlacement.Left,
        LabelsOrientation = System.Windows.Controls.Orientation.Vertical,
        StyleKey = request.AnnotationStyle,
    });
}

And this

<Style x:Key="VerticalLineAnnotationStyle" TargetType="{x:Type s:VerticalLineAnnotation}">
    <Setter Property="Stroke" Value="Orange" />
    <Setter Property="StrokeThickness" Value="2" />
    <Setter Property="FontSize" Value="12" />
    <Setter Property="FontWeight" Value="Bold" />
</Style>

Why I can’t see Label?