The cd command changes the working directory. Called with no argument, cd will bring you to your $HOME directory. Note that cd changes the current working directory of only. Therefore, your plotting program will still be in the previous directory. To get around this difficulty, you only have to define a macro as follows, if your plotting program supports cd:
macro Cd 1 pmode cd "$1" &cd $1 stop alias cd Cd
cd filename
cd cd /nazgul/users/fulano
&, pwd, alias