GraphML output for Solr graph traversals was introduced in version 6.1 (https://issues.apache.org/jira/browse/SOLR-8972).
However, the output is missing some required information as described by the GraphML specification, aka GraphML Primer (http://graphml.graphdrawing.org/primer/graphml-primer.html).
Specifically the attributes are missing the corresponding declarations using the <key> element, as described in http://graphml.graphdrawing.org/primer/graphml-primer.html#Attributes.
Solr's output includes the GraphML-Attribute values, defined using the <data> elements inside nodes and links, but is missing the corresponding declaration of the attributes as <key> elements (including id, name and type) that should precede the graph definition.
This is how it should look like:
And this is how it currently looks like (note the missing <key> elements):