Add a slider on the toolbar which would allow changing the zoom level on the diagram. This should also be controllable more easily (for example, by using shift + mouse wheel scroll).
Personally, I want this less for zooming out and more for zooming in. It's sometimes convenient to have a larger display of a specific area.
I know some people feel that a zoom is a terrible idea because it will encourage people to create huge diagrams. Personally, I doubt that (since you can't work on zoomed out code and zooming out and in repeatedly isn't that convenient), but I don't mind if people do that. It's their code (as long as I don't have to work on it later).
If people really insist, this can also be prevented by setting the maximum value of the slider to 1, so that people can't zoom out, but I doubt this would have any value.
The zoom should snap to 1 when you get near it and should center on the mouse cursor.