OmniGraffle Professional has manual guides to help you accurately lay out your drawings.
To add a new guide either use the Format->Guides menu, or click on a ruler (not while editing text) and drag off a new guide.
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To remove a guide, drag it off the canvas.
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The guides default to aligning centers of graphics. Use the alignment widget on the Alignment inspector to change between the center and either of the edges.
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Manual Guides