Layers
The layers of a canvas are listed in the utility drawer. To open the utility drawer, select it from the View menu or click its toolbar icon. In OmniGraffle Professional, click the Canvases tab at the top of the drawer; in OmniGraffle Standard, click the Layers tab.
Click the disclosure triangle next to a canvas to see its layers. A master canvas may have only one layer, while a regular canvas can have as many layers as you like. Layers are transparent, so behind the top layer's contents you see the second layer's contents, and so on to the contents of the bottom layer.
Each layer in the list has a small preview of its contents, a title, and three icons. The layer you are currently editing has a pencil icon 
to its left. To edit a different layer, click just to the left of its preview and the pencil icon moves there. Any layers on which you have selected objects display a selection rectangle icon 
.
to its left. To edit a different layer, click just to the left of its preview and the pencil icon moves there. Any layers on which you have selected objects display a selection rectangle icon . To make a layer visible or invisible, click its eye icon 
. To set whether a layer appears when the canvas is printed, click its printer icon 
. To lock or unlock a layer, click its padlock icon 
. Locking a layer prevents its contents from being changed.
You can rearrange layers by dragging them in the list. You can even drag a layer to a different canvas or a completely different document to copy it there.
To add a new layer, first select an existing layer in the canvas where you'd like to add it. Then click the plus button at the bottom of the canvases drawer.
To remove a layer, select it and then click the minus button at the bottom of the canvas drawer. Each canvas needs at least one layer, so you can't remove the last one.