return menu tree for graphs (recursive)

def graph_menus():
    '''return menu tree for graphs (recursive)'''
    ret = MPMenuSubMenu('Graphs', [])
    for i in range(len(mestate.graphs)):
        g = mestate.graphs[i]
        path = g.name.split('/')
        name = path[-1]
        path = path[:-1]
        ret.add_to_submenu(path, MPMenuItem(name, name, '# graph :%u' % i))
    return ret