You may see an error similar to the following in the web GUI when attempting to open Nagvis:
Graphviz call failed (1): Format: "cmapx" not recognized. Use one of:<br /><br >Command was: "/usr/bin/neato -Tcmapx /opt/monitor/op5/nagvis_ls/var/foo.dot 2>&1"
Most likely, the only thing required is to reinstall your "graphviz" package. Try:
# yum reinstall graphviz