hscale=auto;
[layout=overlap]{
    a->b: msg; a->b: msg;
    a->b: msg; a->b: msg;
    a->b: msg; a->b: msg;
} [vertical_ident=bottom] {
    b->c [color=red];
    b->c [color=red];
} [vertical_ident=middle] {
    c->d [color=green];
    c->d [color=green];
} [vertical_ident=top] {
    d->e [color=blue];
    d->e [color=blue];
};    