Overlays one plot object on another.
procedure overlay ( base_id  : graphic, transform_id  : graphic )
The base plot of the overlay chain of plots. The object referenced must support overlays.transform_id
The graphical object to be overlaid on the base_id plot. The object must be one of the Transform objects; that is, an object that can go through a transformation to become part of the base_id plot. Examples of Transform objects include contour plots, XY plots, vector plots, and streamline plots.
The overlay procedure is used to set the transform_id object so it will be drawn over the base_id plot. The base_id plot must have the same workstation parent as the transform_id object, and it cannot already be an overlaid plot itself. The transform_id object will become a plot member of the base_id plot.
The base plot will provide the coordinate transformation and data boundaries that together determine where the data coordinate space belonging to the overlay will appear, and how much of it will be visible. In addition, if the transform is a plot object, the base plot assumes management responsibilities for the plot object's plot members if it has any.
It is possible to overlay multiple plots on the same base plot, but you need to call overlay for each plot you want to overlay. When a new plot is overlaid on a base plot that already has other overlaid members, the new plot will be drawn on top of all of the plots currently in the base plot's overlay list.
Note that map plots cannot become overlay plots, although they can serve as base plots.
Once you overlay transform_id plot, you can no longer draw it separately. You can only draw the base_id plot, which causes the base plot to be drawn along with all the plots that have been overlaid on it.