Adding key or legend?

[steveroush]

Is this what you are after?

digraph l {
  subgraph clusterMain {
	graph [labelloc="b" labeljust="r" label=<
	<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
	<TR><TD>left 1</TD><TD>right 1</TD></TR>
	<TR><TD>left 2</TD><TD>right 2</TD></TR>	</TABLE>>];
  a -> b -> c
  "more nodes and edges"
}
}