This is most likely a Visio problem (Visio's SVG support is really poor). Please check the exported SVG in an application with decent SVG support such as Inkscape, Firefox, or even Emacs. If your file is broken in one of these applications, too, please upload the original GraphML and the SVG here, so we can investigate the problem.
Uploading can be done using the link tool in the toolbar (first row, second to last icon).