Thanks for the great support! 

I am now "one step further"...

I changed my source like this:

XAML:

<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mvvm="http://prismlibrary.com/"
xmlns:ig="http://schemas.infragistics.com/xaml"
x:Name="UserControlChartCard" x:Class="TorqueLink.Presentation.Views.ChartCard"
mvvm:ViewModelLocator.AutoWireViewModel="True"
mc:Ignorable="d" Height="300" Width="500">
<Grid x:Name="LayoutRoot" Margin="0" Background="White">
<ig:XamDataChart x:Name="SignalChart" Margin="10" DataContext="{Binding ChartValues}" HorizontalZoomable="True" VerticalZoomable="True">
<ig:XamDataChart.Series>
<ig:LineSeries ItemsSource="{Binding DataContext, ElementName=SignalChart}" ValueMemberPath="SignalData" XAxis="{Binding ElementName=xAxis}" YAxis="{Binding ElementName=yAxis}"/>
</ig:XamDataChart.Series>
<ig:XamDataChart.Axes>
<ig:CategoryDateTimeXAxis x:Name="xAxis" ItemsSource="{Binding DataContext, ElementName=SignalChart}" DateTimeMemberPath="SignalDateTime" />
<ig:NumericYAxis x:Name="yAxis"/>
</ig:XamDataChart.Axes>
</ig:XamDataChart>
</Grid>
</UserControl>

ViewModel:

public class ViewModelChart : ViewModelBase
{
public delegate void SetDataDeviceObjectDelegate(DataDeviceObject dataDeviceObject);
public static Dispatcher CurrentDispatcher;
public static ItemAdderDelegate SignalItemAdderDelegate;
public delegate void ItemAdderDelegate(Signal item);

[NotNull] private readonly IDeviceDataManager _deviceDataManager;

public ViewModelChart([NotNull] IDeviceDataManager deviceDataManager)
{
if (deviceDataManager == null) throw new ArgumentNullException(nameof(deviceDataManager));
_deviceDataManager = deviceDataManager;
CurrentDispatcher = Dispatcher.CurrentDispatcher;
SignalItemAdderDelegate = ItemAdder;

//TODO: Use EventAggregator
ChartAction = StartSignalChecker;
}

[NotNull] public ObservableCollection<Signal> ChartValues { get; } = new ObservableCollection<Signal>();

public void StartSignalChecker()
{
if (_deviceDataManager.RegisteredDevices.Count <= 0) return;

IDeviceCommunicator deviceCommunicator = _deviceDataManager.GetCommunicator();
if (deviceCommunicator.ConnectionState != DeviceConnectionState.Connected) deviceCommunicator.Connect();
string deviceName = deviceCommunicator.DeviceName;
SetDataDeviceObjectDelegate delegateDeviceObject = SetDataDeviceObject;
_deviceDataManager.CreateGetDataDeviceObjectThread(delegateDeviceObject, deviceName, 400);
}

public void SetDataDeviceObject(IDataDeviceObject dataDeviceObject)
{
if (dataDeviceObject == null || dataDeviceObject.Data == 0) return;

Signal signal = new Signal {SignalDateTime = DateTime.Now, SignalData = dataDeviceObject.Signal};
CurrentDispatcher.Invoke(DispatcherPriority.Background, SignalItemAdderDelegate, signal);
}
private void ItemAdder(Signal item)
{
ChartValues.Add(item);
}

public class Signal : INotifyPropertyChanged
{
private decimal _signalData;
private DateTime _signalDateTime;

public decimal SignalData
{
get { return _signalData; }
set
{
_signalData = value;
OnPropertyChanged();
}
}

public DateTime SignalDateTime
{
get { return _signalDateTime; }
set
{
_signalDateTime = SignalDateTime;
OnPropertyChanged();
}
}

public event PropertyChangedEventHandler PropertyChanged;

protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) { OnPropertyChanged(new PropertyChangedEventArgs(propertyName)); }

protected virtual void OnPropertyChanged(PropertyChangedEventArgs propertyChangedEventArgs) { RaisePropertyChanged()?.Invoke(propertyChangedEventArgs); }

protected Action<PropertyChangedEventArgs> RaisePropertyChanged() { return args => PropertyChanged?.Invoke(this, args); }
}
}

i have nothing in code behind.

Thats what i seen now:

Seems i still do anything wrong? :(

Your SignalDateTime property setter has a typo.  It is setting _signalDateTime to SignalDateTime instead of "value".  Change it to say "_signalDateTime = value;".

ofc yes :D

thx very much!! it works now ... *happy*

Hi Stefan,

Great!  Let me know if you have any further questions.