Oracle build_plan_xml visualizer
I think you'd be better off using the text version of the explain plan output rather than XML, as it gives you a pre-formatted readable ascii representation of the query plan.

eg:

SELECT * FROM TABLE(dbms_xplan.display('PLAN_TABLE','#schema_short_code#/#query_id#','ALL'));


Failing that, if you have the means to do so server-side, you could use Oracle SQL Developer to give you a graphical representation of the query plan - Though this is really just the same as the ASCII version with a few icons.

Also, take a look at $ORACLE_HOME/rdbms/xml/orarep/xplan/xplSch.xsd , $ORACLE_HOME/rdbms/xml/orarep/xplan/xplHtml.xsl and $ORACLE_HOME/rdbms/xml/xsl/kuplan.xsl - They may help you format your XML output to prettier HTML.