title: Rank attribute is confusing to me
body: <p>Rank attribute on edge has five values "same", "min", "source", "max", "sink".
  Except "same", I have no idea when to use other values.</p>
  
  <pre><code>min
  \begin{dotpic}
    rankdir=LR;
    size="7,5";
    node[shape=circle];
    C-&gt;A;
    {rank=min;A;B}
    B-&gt;D
    A-&gt;B;
  \end{dotpic}
  
  max
  \begin{dotpic}
    rankdir=LR;
    size="7,5";
    node[shape=circle];
    C-&gt;A;
    {rank=max;A;B}
    B-&gt;D
    A-&gt;B;
  \end{dotpic}
  
  source
  \begin{dotpic}
    rankdir=LR;
    size="7,5";
    node[shape=circle];
    C-&gt;A;
    {rank=source;A;B}
    B-&gt;D
    A-&gt;B;
  \end{dotpic}
  
  sink
  \begin{dotpic}
    rankdir=LR;
    size="7,5";
    node[shape=circle];
    C-&gt;A;
    {rank=sink;A;B}
    B-&gt;D
    A-&gt;B;
  \end{dotpic}
  </code></pre>
  
  <p>With test on my vim environment, I can realize there is some difference btw these values.
  But don't know exactly what they are for.</p>