OmniGraffle is in many ways an excellent piece of software, but its automatic layout engine (Graphviz-based) isn't able to produce a conventional org-chart layout. (It offers orthogonal connectors, but can't allocate space orthogonally, and the results can be quite eccentric).

There are other tools available for OS X which can generate conventional org-charts a bit more easily. Xmind, for example, which has a free version, might be worth looking at, otherwise, perhaps the various SmartArt options in MS Office.

(Orgcharts have now been de-emphasised in the marketing presentation of OmniGraffle).

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