Give the plot a slightly larger z value than the surface. Well, not exactly z value, really, but a coordinate which is slightly closer along the viewing angle than the surface itself is.
When you have a line and a surface at the same distance along the projection, then if you are in OpenGL (not unlikely for surface plots) then OpenGL has its own ideas of which should be drawn first, and nothing you can do by using uistack() can alter what OpenGL will do. So you separate them slightly so that one is "closer".
