Since we need to add the graphml visualization to our software, 

I am wondering if the parser part of BaseX can be used independently?

To have an idea, look at the following GraphML from [1],

<graphml xmlns="http://graphml.graphdrawing.org/xmlns" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns                                graphml+svg.xsd">

  <key id="k0" for="node">

    <default>

      <svg:svg width="5cm" height="4cm" version="1.1">

        <svg:desc>Default graphical representation for nodes</svg:desc>

        <svg:rect x="0.5cm" y="0.5cm" width="2cm" height="1cm"/>

      </svg:svg>

    </default>

  </key>

  <key id="k1" for="edge">

    <desc>Graphical representation for edges</desc>

  </key>

  <graph edgedefault="directed">

    <node id="n0">

      <data key="k0">

        <svg:svg width="4cm" height="8cm" version="1.1">

          <svg:ellipse cx="2cm" cy="4cm" rx="2cm" ry="1cm"/>

        </svg:svg>

      </data>

    </node>

    <node id="n1"/>

    <edge source="n0" target="n1">

      <data key="k1">

        <svg:svg width="12cm" height="4cm" viewBox="0 0 1200 400">

          <svg:line x1="100" y1="300" x2="300" y2="100" stroke-width="5"/>

        </svg:svg>

      </data>

    </edge>

  </graph>

</graphml>