By using the Smart Guides, you really do not have to care about where your shapes are placed relative to the canvas grid. However, if you create diagrams that involve shapes that have to be correctly sized to match the "real world", you can get a lot of help from OmniGraffle by enabling the Snap to grid checkbox and the Edges on grid button on the Grid Property inspector.
The following discussion will demonstrate how OmnniGraffle will help you create a floor plan (as seen in the previous picture) for an exhibition your company is holding.
Your company, which sells maritime gear, is arranging an exhibition for your customers. You have invited all your suppliers, and they will exhibit products and goods.
The exhibition is held in a room measuring 20 X 30 meters (65.62 X 98.42 feet).
The first thing you have to set up is a grid that will help you in drawing your diagram. A normal measurement in this regard is letting 1 cm on paper equal 1 m in real life ...