When working in OmniGraffle, you need create non-symmetrical objects and then use them again. For example:
And you need to flip the shape to use it on the other side. The shape can’t be rotated (using CMD-Mouse and drag) to make this happen, it needs to rotated 180 degrees on the vertical or horizontal axis.
Locating this in OmniGraffle is a bit tricky. Yo might expect it to be Edit, Shapes menu but it’s actually in the inspector.
In most things OmniGraffle is sublime, but maybe it’s a MAC thing for it to be in the floating menu.