Rulers

The rulers help you position objects on the canvas, and contain tools for dealing with the scale of a document. To see the rulers, select them from the View menu or click their toolbar icon.
As you drag objects around the canvas, guides appear on the ruler to help you find just the right placement. You can also create manual guides from the ruler: click and hold in the top or side ruler, then drag to the canvas area.
By default, the very upper-left corner of a canvas is its origin (that is, the point where the rulers' measurements start from, where the coordinates are 0,0). To change the origin, drag the blue cross from the upper left where the rulers meet to wherever you would like it to be, or enter values in the two Origin fields. The measurements in the Geometry inspector are based on this origin point.
Ruler Units
The Ruler Units pop-up menu is where you set the type of measurement units you'd like to use for the current canvas. All of OmniGraffle's supported units are listed, with their standard abbreviations.
The actual size of your diagram does not change when you change the units; it is merely measured differently. The ruler and the inspectors display measurements in whichever unit you select here.
Units marked with an asterisk (*) can be used as canvas units. This means that when you set up a ruler scale, only these units can be used on the left-side "actual size" part of the equation. Any kind of units can be used on the right-side "theoretical size" part of the scale equation.
Ruler Scale
The Ruler Scale field accepts simple expressions of scale.
You can enter an equation. For example, say you enter 1 cm = 1 m. What was 1 centimeter on the ruler now becomes 1 meter, objects on the canvas that were 2 centimeters wide now claim to be 2 meters wide, and so on. The ruler units setting changes to match the second value in the equation.
You can also enter a ratio. Imagine that you enter 1:12. The ruler units stay the same, but objects on the canvas now claim to be 12 times larger than they were before you changed the scale.
To change the scale back to normal and treat the ruler and objects as "actual size", select Reset Scale from the Ruler Scale field's pop-up menu.
If you have a scale set up, and you do not reset the scale before entering a new one, the objects will change their actual size on the canvas to fit the new scale. For example, imagine you are working in 1 cm = 1 m, and then you change the scale immediately to 2 cm = 1 m. The marks on the ruler become twice as far apart as they were, and the objects on the canvas, staying true to the ruler, grow twice as large as they were.
Text Ruler
When you are editing text, the ruler becomes a standard text ruler. You can style text, change the spacing and alignment, or drag tab stops to and from the ruler.