Data bound constant lines should move if value property is changed

The code I wrote to bind the ConstantLine's Value property:

            ' View chart of difference between actual and normal worktime
            Dim vDiagram As New XYDiagram2D()
            Dim vArgumentDataMember As String = GetPropertyName(Function(wtd As WorkTimeDifference) wtd.Date)
            Dim vValueDataMember As String = GetPropertyName(Function(wtd As WorkTimeDifference) wtd.WorkTimeDifference)

            Dim vDisplayName As String = "Actual - normal worktime"
            Dim vWorkTimeDifferenceBinding As New Binding("WorkTimeDifferenceForChart")
            vWorkTimeDifferenceBinding.Source = Me.DataContext
            Dim vWorkTimeDifferenceSeries As New AreaSeries2D With {.DisplayName = vDisplayName, .ArgumentScaleType = ScaleType.DateTime, .ValueDataMember = vValueDataMember, .ArgumentDataMember = vArgumentDataMember, .Transparency = 0.4, .Label = New SeriesLabel With {.Visible = False}}
            vWorkTimeDifferenceSeries.SetBinding(AreaSeries2D.DataSourceProperty, vWorkTimeDifferenceBinding)
            vDiagram.Series.Add(vWorkTimeDifferenceSeries)

            Dim vAverageBinding As New Binding("AverageWorkTimeDifference")
            vAverageBinding.Source = Me.DataContext
            Dim vAverageLine As New ConstantLine()
            vAverageLine.SetBinding(ConstantLine.ValueProperty, vAverageBinding)
            vAverageLine.Brush = Brushes.Orange
            vAverageLine.ShowBehind = True

            Dim vAverageTitleBinding As New Binding("AverageWorkTimeDifference")
            vAverageTitleBinding.StringFormat = "({0:n3}) Average difference"
            Dim vAverageTitle As New ConstantLineTitle()
            vAverageTitle.SetBinding(ConstantLineTitle.TextProperty, vAverageTitleBinding)
            vAverageTitle.Foreground = Brushes.Orange
            vAverageTitle.Alignment = ConstantLineTitleAlignment.Far
            vAverageLine.Title = vAverageTitle

            ' Default settings
            vDiagram.SeriesTemplate = New LineSeries2D()
            vDiagram.AxisY = New AxisY2D()
            vDiagram.AxisX = New AxisX2D()
            vDiagram.AxisY.ConstantLines.Add(vAverageLine)
            Me.WorkTimeChartControl.Diagram = vDiagram