The Canvas Inspector
The Canvas inspector controls the dimensions and appearance of the drawing area for your graph. To open it, click the third tab in the inspector. If the inspector isn’t open, choose Canvas from the Inspectors menu.
Click the Color well to open the standard Mac color panel and choose a background color for your graph. You can even choose partially transparent colors, if you need to export a copy of your graph to overlay on top of something else.
If you must add shadows to your graph, you can do so with the Shadows slider. But take heed: information visualization experts frown upon such frippery.
The Canvas Size fields show the dimensions of your window, in points. You can just resize the window, of course, but these fields give you precise control in case you are aiming for a very particular size. If you need to make a much bigger or much smaller final product than your screen will accommodate, you can export as PDF and then use a graphics application to scale it.
The Margins section controls the area of the canvas outside your graph. For the most part, you can let OmniGraphSketcher automatically set your margins for you, based on the size and distance of your axis labels. But if you need more space (to add annotations, for instance), you can deselect the Automatic checkbox and enter specific values. (You could also drag the magenta guides that appear when you put the mouse pointer over one of the margins.)
The Window Transparancy slider controls the opacity of your canvas so that you can trace another graph or compare the graph you've drawn to another window behind it. Move the slider to the left for a clear background and to the right for an opaque background.