Theory: OmniGraffle creates automatically a diagram from any OminOutliner file – just great for creating and iterating/changing sitemaps. You open the OO document in OG and you will be asked to select a certain diagram-style from a list that contains pre-defined or self-created diagram-styles (which is essentially a small sample of the model-diagram that you save in a particular format in a particular folder).
Praxis: This works fine if the diagram is rather small/simple AND if you are happy with either strict top-to-bottom or left-to-right arrangement. Auto-Diagramming is useless if you need to assign different arrangement directions for individual levels (a necessity to fit a complex sitemap on one page). If you create a diagram-style with both horizontally and vertically arranged layers, you will realise that these ‘subtleties’ will be ignored. Applying this style to a diagram will force all layers to follow (usually?) a top-to bottom arrangement. You will end up moving objects around, fiddling with automatically created connection lines and spending probably more time changing the diagram than you would by just creating one branch manually and copying/pasting/editing this branch to the remaining sections of the sitemap.