title: Visualizing graph/network with 3 layeres (tripartite) in R/igraph
body: <p>I have a "layered" network, with 3 layers, let's say parents(P), children(C), grandchildren(G). The edges are always directed and towards a younger generation (either patent->child, child->grandchild or parent->grandchild). No edges between vertices in the same generation. 
  The graph is represented by 3 edge lists (P_C, C_G, P_C). A short example is given bellow.</p>
  
  <p>1) What is the proper term for this sort of graph/network? tripartite graph? As such, I suppose it is a particular case because of the lack of backward connections. </p>
  
  <p>2) How do I represent it as a graph object in R (igraph)? </p>
  
  <p>3) Can I plot the graph in a way that depicts the "layers", with all the vertices for each group (P,C,GC) aligned at the same x coordinates, going from P on the left, C in the middle and GC on the rigth ?</p>
  
  <p>4) Can I check for graph isomorphisms between graphs with this structure, thaking into account the layered nature of the data. (I know for regular graphs it would be the graph.isomorphic() function). </p>
  
  <pre><code>edge_P_C &lt;- read.table(text="P C
  A B
  A C", header=T)
  
  edge_C_G &lt;- read.table(text="C G
  B D
  B E
  C F", header=T)
  
  edge_P_G &lt;- read.table(text="P G
  A G", header=T)
  </code></pre>