PR2 rqt_plot has the error TopicCompleter.update_topics(): could not get message class

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I have written a publisher in PR2's computer to publish message whose type is "beginner_tutorials/Num" on topic "/chatter2". Then I want to plot the data from this topic. I cannot use rqt_plot in PR2's computer because it does not support graphics. So I open a terminal in my own computer(which of course supports graphics) and export ROS_MASTER_URI=[PR2 computer's address] so that I can communicate with PR2's roscore. In this local terminal, I use rqt_plot to plot the data like: rqt_plot /chatter2/data, but there is an error:

TopicCompleter.update_topics(): could not get message class for topic type "beginner_tutorials/Num" on topic "/chatter2"

It seems that rqt_plot cannot subscribe to the topic. But curiously, indeed I can "rostopic list" this topic in the same terminal. So how to let rqt_plot subscribe successfully to the topic and plot my data?