There is probably this bug
--- viper/viper_dolfin.py 2011-12-07 12:56:29.000000000 +0100
+++ viper/viper_dolfin.py 2013-07-11 21:55:21.000000000 +0200
@@ -269,7 +269,7 @@
vertex_values = vertices.array()
vertex_values[:] = 0
con20 = self.mesh.topology()(dim,0)
- for facet in xrange(self.mesh.num_faces()):
+ for facet in xrange(self.mesh.num_facets()):
if values[facet]:
vertex_values[con20(facet)] = values[facet]
self.plot_meshfunction(vertices, *args, **kwargs)
But I can't test it on my machine. Viper is not maintained anymore and FEniCS with Viper won't be released. So you can apply it locally.