Graphviz lamport diagrams (borrowed from palvaro's molly)

lamport.dot

digraph spacetime {
    rankdir=TD
    splines=line
    // outputorder=nodesfirst
    subgraph cluster_proc_nodes {
        label="";
        proc_a [label="Process a",group="a"];
        proc_b [label="Process b",group="b"];
    }

  node_a_1[label="1", group="a"];
  node_a_2[label="2", group="a", shape="point"];
  node_a_3[label="3", group="a", shape="point"];
  node_a_4[label="4", group="a"];

  node_b_1[label="1", group="b", shape="point"];
  node_b_2[label="2", group="b"];
  node_b_3[label="3", group="b"];
  node_b_4[label="4", group="b", shape="point"];

  node_a_1 ->node_b_2[constraint=false, label="req"];
  node_b_3 ->node_a_4[constraint=false, label="res"];

    edge[weight=2, arrowhead=none, color=gray75, fillcolor=gray75];
    proc_a -> node_a_1 -> node_a_2 -> node_a_3 -> node_a_4 ;
    edge[weight=2, arrowhead=none, color=gray75, fillcolor=gray75];
    proc_b -> node_b_1 -> node_b_2 -> node_b_3 -> node_b_4 ;
}

lamport.svg