How to center Graphiz transition label?

how can I center label in graphviz ?

digraph "toto" {
graph ["rankdir"="TB"]
node ["shape"="box"]
"BROUILLON" ["label"="Brouillon","margin"="0.4,0.0"]
"A_VALIDER_RH" ["label"="À valider par la RH","margin"="0.4,0.0","color"="red"]
"A_VALIDER_RH" -> "BROUILLON" ["label"="Refuser"]
"BROUILLON" -> "A_VALIDER_RH" ["label"="Soumettre"]
}

I have:

expected:

Solution

You will need to use extra nodes for the labels:

digraph "toto" 
{
    // graph[ "rankdir" = "TB" ]
    
    // nodes
    node[ shape = box ];
    BROUILLON[ label= "Brouillon", margin = "0.4,0.0" ];
    A_VALIDER_RH[ label = "À valider par la RH", margin = "0.4,0.0", color = red ];
    node[ shape = plaintext ];
    n1[ label = "Soumettre" ];
    n2[ label = "Refuser" ];

    // edges
    BROUILLON -> n1 ->  A_VALIDER_RH;
    BROUILLON -> n2 -> A_VALIDER_RH[ dir = back ];
}

yields