Вы можете использовать LineAnnotation для этой цели.Например,
var annotation = new LineAnnotation();
annotation.Color = OxyColors.Blue;
annotation.MinimumY = 10;
annotation.MaximumY = 40;
annotation.X = 5;
annotation.LineStyle = LineStyle.Solid;
annotation.Type = LineAnnotationType.Vertical;
MyPlotModel.Annotations.Add(annotation);
Пример вывода
![enter image description here](https://i.stack.imgur.com/4o57A.png)