Selecting multiple objects with shift key
 

When I want to select multiple objects while holding the shift key, Omnigraffle will select objects that aren't current selected and de-select objects that are already selected (an XOR operation).

Is there a way to get it to do just an OR function? Ie., I might want to select a couple of objects using click-drag. Then I want to select some more objects using another click-drag which partly overlaps the previous set of objects--but I don't want any of the previous objects de-selected.