YMMV, but this should export a PNG of a zenmap topology from a Nmap XML file

cli_zenmap_png.py

#!/usr/bin/env python

import sys

if len(sys.argv) != 4:
    print """{0} - Output a PNG from Nmap XML
    Usage: {0} <scan.xml> <out.png> <width_in_pixels>""".format(sys.argv[0])
    sys.exit(1)

try:
    from zenmapGUI.TopologyPage import *
except ImportError:
    import sys
    sys.path.insert(0,"/edit/me/path/to/zenmap")
    from zenmapGUI.TopologyPage import *

t = TopologyPage(NetworkInventory(sys.argv[1]))
pix = int(sys.argv[3])
t.radialnet.set_allocation((0,0,pix,pix))
t.update_radialnet()
t.radialnet.save_drawing_to_file(sys.argv[2])

It worked for me after replacing the source code of Networkinventry.py of zenmapcore from this link.