digraph Pipeline {
        rankdir=LR
        labeljust="l"
        style=filled
        color=lightgrey
        node [style=filled,fillcolor=white,color=white]
        subgraph clusterApplication {
                label="Application program"
                program [label="Main program",shape=rect]
                callback [label="Callback function"]
# invisible content to match layout with intention:
                edge [style=invis]
                node [style=invis,label="",width=0.01,height=0.01]
                program -> libpad
                libpad -> callback
        }

        subgraph clusterLibrary {
                margin="5,5"
                label="Software library"
                labelloc=b
                node [color=blue,fontcolor=blue]
                function [label="Library function"]
        }

        program -> function [label="calls"]
        function -> callback [label="calls"]
}