I would probably do this by overriding OnRender in your chart UserControl and drawing the path first, then markers second.
See this related question.
In it the poster creates a Geometry to represent a triangle then uses the rendering context passed in to OnRender to draw the triangle repeatedly.
You might find if you have a large number of datapoints this method becomes quite slow. You can experiment instead with multiple small blits of a market bitmap to the rendering context, or using the WriteableBitmap API which provides direct access to bitmap & therefore immediate mode rendering.
Edit: You might also be interested in this answer, which discusses high performance WPF graphics in general.
Best regards,