viewer.getGraphControl().addMouseWheelListener( new MouseWheelListener() {
            @Override
            public void mouseScrolled(final MouseEvent ev) {
                if (( ev.stateMask & SWT.CTRL ) == 0)
                    return;
                                
                if (ev.count > 0) {
                    viewer.getGraphControl().getZoomManager().zoomOut();
                } else if (ev.count < 0) {
                    viewer.getGraphControl().getZoomManager().zoomIn();
                }
            }
        } );