Tutorial: Understanding a Topic: run rxgraph I got a nameError

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Hi all, I am following the ROS tutorial and now come to the Understanding ROS Topics. When I key in rxgraph, it give me

Traceback (most recent call last): File "/home/btian/ros/ros/bin/rxgraph", line 36, in <module> rxgraph.rxgraph_main() File "/home/btian/ros/rx/rxgraph/src/rxgraph/__init__.py", line 61, in rxgraph_main print >> sys.stderr, "Graphviz does not appear to be installed on your system. Please run:\n\n\trosdep install rosgraph\n\nto install the necessary dependencies on your system" NameError: global name 'sys' is not defined

But \n\n\trosdep does not exist. I tried import sys also, does not help. any idea how to get over this?