title: Arranging graphviz clusters
body: <p>I'm trying to generate the following construct:
  I have four graphs. Three of them are supposed to be in the first "row", all vertically aligned by their vertical center. The fourth graph is supposed to be below the other three, in the second "row". It should be horizontally aligned with either the horizontal center of the upper row's middle graph or the horizontal center of the entire upper row. Additionally there will be edges between some members of separate clusters/graphs.
  <img src="https://i.stack.imgur.com/Z8bHQ.png" alt="Here is an example."> Here there are only edges between members of cluster_a and cluster_d, but there may be edges between members of any combination of clusters.</p>