private void RadDiagramRibbon_Loaded(object sender, RoutedEventArgs e)
{
	var toolService = diagram.ServiceLocator.GetService<IToolService>() as ToolService;
	this.shapeTool = (ShapeTool)toolService.ToolList.FirstOrDefault(x => x is ShapeTool);
}

private Point startPoint;
private bool isShapeCreationInProgress;
private MyNode currentShapeModel; // where MyNode derives from NodeViewModelBase and expose an additional Geometry property 
private ShapeTool shapeTool;

private void RadDiagram_PreviewMouseDown(object sender, MouseButtonEventArgs e)
{			
	if (this.shapeTool.IsActive && !this.isShapeCreationInProgress)
	{
		this.isShapeCreationInProgress = true;
		this.startPoint = this.diagram.GetTransformedPoint(e.GetPosition(this.diagram));

		this.currentShapeModel = new MyNode()
		{
			Geometry = this.shapeTool.Geometry,
			Position = this.startPoint,
			Width = 0,
			Height = 0
		};
		var source = (MyGraphSource)this.diagram.GraphSource;
		source.AddNode(this.currentShapeModel);
		e.Handled = true;
	}
}

private void RadDiagram_PreviewMouseMove(object sender, MouseEventArgs e)
{
	if (this.shapeTool.IsActive && this.currentShapeModel != null && this.isShapeCreationInProgress)
	{
		var transformedPoint = this.diagram.GetTransformedPoint(e.GetPosition(this.diagram));
		var width = Math.Abs(this.startPoint.X - transformedPoint.X);
		var height = Math.Abs(this.startPoint.Y - transformedPoint.Y);
		var x = Math.Min(this.startPoint.X, transformedPoint.X);
		var y = Math.Min(this.startPoint.Y, transformedPoint.Y);

		this.currentShapeModel.Width = width;
		this.currentShapeModel.Height = height;
		this.currentShapeModel.Position = new Point(x, y);
		e.Handled = true;
	}
}

private void RadDiagram_PreviewMouseUp(object sender, MouseButtonEventArgs e)
{
	this.isShapeCreationInProgress = false;
	e.Handled = true;
}