An onion (or onion skin) diagram uses concentric circles to represent layered systems. Each component is a ring that builds upon a single, core component, as Figure 11-5 shows. All the shapes you need to create an onion diagram are located on the Blocks stencil, which opens with the Block Diagram template.
Figure 11-5. This onion diagram depicts the layered nature of computer architecture.
To create an onion diagram, start with the outer concept and work your way in. Use a concentric layer shape of the appropriate size to represent each new concept or layer. The concentric layer shapes fit together tightly. As long as you have no more than four layers in your diagram and don't need to resize shapes, the diagram is easy to create. To represent five or more layers, you must manually resize the largest concentric layer shape to fit.
Follow these steps to create an onion diagram:
Perhaps the biggest challenge when working with layer shapes is getting the text to fit. Visio doesn't support text on a curve, so long lines of text can extend beyond a layer's borders. To enlarge a layer so that more text fits, drag a corner selection handle, as Figure 11-6 shows. You'll have to enlarge each layer in the onion diagram a similar amount, and then realign the shapes. Start with the outside layer, and then resize each inner layer. Other techniques for revising shapes are summarized in Table 11-4.
Figure 11-6. To change the radius of a concentric layer, drag a selection handle to resize the layer as needed.
Table 11-4. Revising Concentric Layer Shapes
Align onion layers
Select all the layer shapes, and then choose Shape, Align Shapes. Choose a center alignment option, and then click OK.
Change the thickness of a concentric layer
Drag the control handle on the shape's inside rim to the thickness you want. If you don't know which control handle to use, pause the pointer over a control handle to display a ScreenTip.
Move the text label on a layer
Drag the control handle in the middle of the layer. If you have trouble dragging the handle, disable snapping— Choose Tools, Snap & Glue, clear the Snap check box, and then click OK.
Text doesn't fit in an onion layer.
If you want to add more text than fits within the curves of an onion layer, you have a couple of options. Although curved text isn't one of those options, you can try the following:
Although onion diagrams are not especially complex, you can represent complex ideas by segmenting a layer. The partial layer shapes (Partial Layer 1, Partial Layer 2, and so on) on the Blocks stencil are designed to fit on top of the concentric layer shapes, as Figure 11-7 shows. You can use them to represent subcomponents of a larger system.
Figure 11-7. Two Partial Layer 2 shapes suggest partitions or components on top of a Concentric Layer 2 shape.
Follow these steps to divide a layer into sections:
If the partial layer shape is equal in size to the concentric layer you want to divide, the shape snaps into place. The endpoints turn red when they are connected.