Group multiple namespaces in rqt_graph [closed]

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Hello there!

I'm using ROS Hydro, and I have a little issue which I'll demonstrate with this launch file. I'm relatively new to ROS and I'm not sure if this can be done at all.

<launch>
  <group ns="turtle">
    <node pkg="turtlesim" type="turtlesim_node" name="turtlesim_node" >
      <remap from="turtle1/cmd_vel" to="cmd_vel" />
    </node>
  </group>

  <group ns="control">
    <node pkg="rostopic" type="rostopic" name="publisher1" args="pub -r 1 /turtle/cmd_vel geometry_msgs/Twist -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]'" />
    <node pkg="rostopic" type="rostopic" name="publisher2" args="pub -r 1 cmd_vel geometry_msgs/Twist -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]'" />
  </group>

  <node pkg="rqt_graph" type="rqt_graph" name="rqt_graph" />
</launch>

The rqt_graph shows two namespaces, with publisher1 correctly grouped into the "control" namespace (if "Group namespaces" is checked). Now, if I remove publisher2 from the launch file and launch again, publisher1 doesn't get grouped into the "control" namespace (it doesn't show the namespace box around it).

Is there something I don't understand here, or is this a bug?

Thank you in advance!