Often its convenient to prepare a diagram with multiple overlays to illustrate multiple aspects of the same topic. In dia(1) this is usually done by creating the overlays as layers and then controlling the visibility of the layers when exporting the image.
To make this task easier, PlainDoc supports specification of the layers using special tag:
<<dia: file,posspec,sizespec,trimspec:layer1,layer2: Legenda>>
This is almost the same tag as the img, however with the twist that layers are specified between first and second colon. Use comma to separate layer names if you have multiple. See the above section on images for description of other specs.