Gets or sets a value that determines the z-order "depth" of this
item relative to other graphic objects.
Note that this controls the z-order with respect to
other elements such as CurveItem's, Axis
objects, etc. The order of GraphObj objects having
the same ZOrder value is controlled by their order in
the GraphObjList. The first GraphObj
in the list is drawn in front of other GraphObj
objects having the same ZOrder value.
