Changing how tools work with the Drawing Tools preferences
Drawing Tools is the second preference pane, accessible by choosing Preferences from the OmniGraffle menu.
You can drag tools around in the list to reorder them in the palette. Move the separator line to change where the expansion button appears in the toolbar; move it to the end of the list to get rid of the button entirely.
Double-click a tool's hot key to change it. A hot key activates a tool for only as long as you hold the key down; once you release the key, the previously active tool becomes active again.
Inspect a tool By default, in order to inspect a tool you need to Option-click it. If you prefer, you can make it take just one normal click to inspect a tool.
Keep tool active By default, it takes one click to temporarily activate a tool, and two clicks to make a tool "stick" in an active state. You can choose to make tools stick after only one click, or make it so that every tool selection is temporary.
Line Creation When you are creating a line, clicking an object will always connect the line to the object and end the line. If you click in an empty area of the canvas, though, you might want to end the line there or you might want just to add a point there and continue creating the line. Use this preference setting to indicate which you prefer.
Line Editing When you double-click a line, you might prefer to add a new point to the line, or you might prefer to add a label. Choose one; Option-double-click will do the other.