Sometimes you just want to model a graph in XML.

The most basic kind:

<graph>
   <vertex id="a"/>
   <vertex id="b"/>
   <edge from="a" to="b"/>
   <edge from="b" to="a"/>
</graph>

You could also "type" your edges/vertices with element names:

<graph>
   <akind id="a"/>
   <bkind id="b"/>
   <parent-to-child from="a" to="b"/>
   <child-to-parent from="b" to="a"/>
</graph>