ocamlgraph-viewer: command not found

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 Debian possible default full paths are:

/usr/bin/ocamlgraph-viewer - from package: ocamlgraph-editor

You may install the required package with command apt-get install {package}

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