ROS中执行rosrun rqt_graph报错 No module named 'rospkg'

环境：ubuntu18.04 + ROS Melodic
执行命令ros run rqt_graph rqt_graph报错：

Traceback (most recent call last):
  File "/opt/ros/melodic/lib/rqt_graph/rqt_graph", line 5, in <module>
    from rqt_gui.main import Main
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/main.py", line 38, in <module>
    import rospy
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/__init__.py", line 49, in <module>
    from .client import spin, myargv, init_node, \
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/client.py", line 52, in <module>
    import roslib
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslib/__init__.py", line 50, in <module>
    from roslib.launcher import load_manifest
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslib/launcher.py", line 42, in <module>
    import rospkg
ModuleNotFoundError: No module named 'rospkg'

安装rospkg包：

pip install rospkg

安装完成后再次运行ros run rqt_graph rqt_graph，继续报错No module named ‘pydot’,
安装pydot：

pip install pydot

安装成功后，运行ros run rqt_graph rqt_graph后：

参考博文

ROS执行rosrun rqt_graph报错No module named ‘catkin_pkg’