A best effort service to comment code and learn about this lab.
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Install network simulator $sudo apt install ns2
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Download xgraph from http://www.xgraph.org/linux/index.html for appropriate Ubuntu on your machine. 
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Untar the downloaded file : $sudo tar -C /bin/xgraph/ -xvf xgraph_4.38_linux64.tar.gz
 NOTE:
 a. I have untarred this to my /bin/xgraph/ directory(of which the xgraph dir is once which Ive mkdir'd).You can choose to untar it to your desired location by replacing /bin/xgraph to your destination dir path.
 
 b. This untared dir has a bin dir which contains the xgraph executable which we need to add to .bashrc
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Navigate to home using directory and open .bashrc file (.bashrc is a hidden file and hence may not show up in your file-explorer,either options->show hidden files or open it on VIM via the terminal). 
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Create Alias for the command , add the below line into bashrc file 
 alias xgraph='path_to_the_untarred_dir/bin/xgraph'
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Run the below command to reflect changes made to bashrc : 
 $ source ~/.bashrc
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Have fun with tcl.`