GDSPLOT supports special fill patterns for contact and via layers. This is a "transparent" fill that enables the designer to see through a contact. It is especially useful for plotting devices that have "stacked" contacts or vias.
The three types of contact fills are:
 An x through the square or rectangular contact.
 A plus sign.
 A diamond.
In the lower row you can see how these appear when vias or contacts are stacked.