basic shapes - no subscriber seen in rqt_graph

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hi, after I went through the ROS Tutorials for beginners, I started to learn simulator - RViz. when I went through this page: http://wiki.ros.org/rviz/Tutorials/Ma...

but at the last step, the basic shapes does not appear in my RViz's UI. it has been solved at my answer.But I have another question, why there is no listerer subscribed to the topic /visualization_marker? if we use rqt_graph to see it?

I run "$ rostopic echo /visualization_marker" and there exists messages updating at 1Hz. I also attached the screen shot below.

But I have another question, why there is no listerer subscribed to the topic /visualization_marker? if we use rqt_graph to see it?

Thank you for any help!

---------below is the info of the teminal running rviz:-------------

exbot@ubuntu:~$ rosrun rviz rviz libGL error: failed to load driver: vboxvideo libGL error: Try again with LIBGL_DEBUG=verbose for more details.