pltek: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/pltek - from package: plplot

You may install the required package with command yum install {package}
Error in other languages:
pltek: bevel nie gevind nie
pltek: comandă negăsită
pltek: comando não encontrado
pltek: commande introuvable
pltek: command not found
pltek: käsku ei ole
pltek: không tìm thấy lệnh
pltek: komanda nerasta
pltek: Kommando nicht gefunden.
pltek: kommandot finns inte
pltek: komut yok
pltek: nie znaleziono polecenia
pltek: níor aimsíodh an t-ordú
pltek: no se encontró la orden
pltek: no s'ha trobat l'ordre
pltek: opdracht niet gevonden
pltek: parancs nem található
pltek: perintah tidak ditemukan
pltek: príkaz nenájdený
pltek: příkaz nenalezen
pltek: команда не найдена
pltek: командата не е открита
pltek: 命令找不到
pltek: コマンドが見つかりません