We had an interesting tech-support question recently:
“I have segment that starts at (0,0) and ends at (0,10). Call it AC. It measures 4.45784(..) inches. The units are set for inches. Why does it not measure 10 inches?”

The answer is that the coordinate system is not locked to a particular scale; in fact, you can easily change the scale of a default coordinate system by dragging one of the tick numbers, and if you change the Grid Form to Rectangular, you can change the scale of each axis independently of the other. The result of changing the scale of the coordinate system is that the coordinate system's units no longer match your preferred distance units.

When you first define a coordinate system using Graph | Define Coordinate System, the system begins with a scale of 1 cm or 1 inch depending on your preferred units. (If your preferred units are pixels, the coordinate system begins with a scale of 10 pixels.) As long as you don't change the scale of either axis, distances measured in your preferred units (of cm or inches) will match the coordinate measurements. But once you change the scale, this connection is broken.

In situations where this creates a problem, there are two ways to resolve it: (a) you can make your measurements in coordinate units rather than cm or inches, or (b) you can create the coordinate system in a way that locks it to a scale of 1 cm or 1 inch, and prevents any rescaling of the axes.

(a) To make distance measurements in coordinate units, use this Measure menu command: Measure | Coordinate Distance. To make other measurements in coordinate units, you can use the custom tools provided here: http://sketchexchange.keypress.com/f...in-Graph-Units

(b) To create a coordinate system with a scale locked at 1 cm, create a distance parameter of 1 cm using Number | New Parameter. (You must click the Distance radio button to make this a distance parameter.) Then with the parameter selected, choose Graph | Define Unit Distance. The axes of the resulting coordinate system cannot be scaled (except by changing the value of the parameter).

For most purposes, solution (a) is the preferred one, because it gives you more flexibility as you develop and modify your sketch.