Print the Congestion Window Graph in NS2

Congestion window size is a major factor in predicting the performance of a TCP Network. NS2 handles the Congestion window like a variable cwnd_ and it can be printed to a graph and as well printed as values at various intervals of time.
The following tcl script will let you understand the working of the network (TCP Reno Protocol) and also helps you in plotting the characteristics of Congestion window value.
Xgraph is used to plot the congestion window. To install xgraph, one can use the ns-allinone-2.3x package or directly install using
yum install xgraph (fedora) and
sudo apt-get install xgraph (Ubuntu, Linux Mint and Debian families)
Each and every line of the code is explained with comments. Still any doubts, please post your queries in the comment section.
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#create simulator
set ns [new Simulator]        
#to create nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
set n5 [$ns node]

# to create the link between the nodes with bandwidth, delay and queue
$ns duplex-link $n0 $n2   2Mb  10ms DropTail
$ns duplex-link $n1 $n2   2Mb  10ms DropTail
$ns duplex-link $n2 $n3 0.3Mb 200ms DropTail
$ns duplex-link $n3 $n4 0.5Mb  40ms DropTail
$ns duplex-link $n3 $n5 0.5Mb  30ms DropTail

# Sending node is 0 with agent as Reno Agent
set tcp1 [new Agent/TCP/Reno]
$ns attach-agent $n0 $tcp1

# receiving (sink) node is n4
set sink1 [new Agent/TCPSink]
$ns attach-agent $n4 $sink1

# establish the traffic between the source and sink
$ns connect $tcp1 $sink1

# Setup a FTP traffic generator on "tcp1"
set ftp1 [new Application/FTP]
$ftp1 attach-agent $tcp1
$ftp1 set type_ FTP               

# start/stop the traffic
$ns at 0.1   "$ftp1 start"
$ns at 40.0 "$ftp1 stop"

# Set simulation end time
$ns at 50.0 "finish"            
# procedure to plot the congestion window
proc plotWindow {tcpSource outfile} {
   global ns
   set now [$ns now]
   set cwnd [$tcpSource set cwnd_]

# the data is recorded in a file called congestion.xg (this can be plotted # using xgraph or gnuplot. this example uses xgraph to plot the cwnd_
   puts  $outfile  "$now $cwnd"
   $ns at [expr $now+0.1] "plotWindow $tcpSource  $outfile"
}

set outfile [open  "congestion.xg"  w]
$ns  at  0.0  "plotWindow $tcp1  $outfile"
proc finish {} {
exec xgraph congestion.xg -geometry 300x300 &
   exit 0
}
# Run simulation 
$ns run
Here is the output looks like
Run the above file using "ns congestion.tcl"