how to plot element in poseArray?

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Hi, I am trying trying to plot each pose data in poseArray (/myPoseArray/poses[0]/postion), but in rqt plot it shows nothing. I can see the data in topic monitor. It looks fine. I try to use rostopic echo, I can get all the array data print out with /myPoseArray/poses . But I can't print specific one using /myPoseArray/poses[0]/postion or /myPoseArray/poses[0]/postion/x . Any ideas what could go wrong? Thanks in advance.