plot(P.comparability_graph()) cannot run
P = Poset(DiGraph({'a':['b'],'b':['c'],'c':['d']}), facade = True);
P
P.comparability_graph()------ cannot run
........ dot -Txdot file.dot | dot2tex > file.tex .............
This works for me, could you please tell us which version of Sage are you using, with which build, and on which OS and architecture ?