Currently the project web site of the reasearch group which defined GraphXML is being moved, therefore it can be a tad cumbersome to get hold of the relevant documents. Fortunately, the format (at least the subset as used by VanillaVista) is very simple:

<GraphXML>
    <graph>
        <node name="a">
            <label>a</label> 
        </node>
        <node name="b">
            <label>b</label> 
        </node>
        <node name="c">
            <label>c</label> 
        </node>
        <edge source="a" target="b" /> 
        <edge source="a" target="c" /> 
        <edge source="c" target="a" /> 
    </graph>
</GraphXML>