Do you want to draw a single triangle, or a triangle on every data-point?
If a triangle per-datapoint, please see the Triangle PointMarker type described here.
If a single triangle e.g. in an annotation, you will need to template BoxAnnotation with a custom path or control which overrides OnRender. E.g. create a UIElement which renders a triangle and just set as the child in a BoxAnnotation control template.
Hope this helps!
Please login first to submit.