web/web_graph_export_png/view/web_graph_export_png.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>
  <data>

    <template id="assets_backend"
              name="web_graph_export_png assets"
              inherit_id="web.assets_backend">
      <xpath expr="." position="inside">
        <script type="text/javascript"
                src="/web_graph_export_png/static/src/js/web_graph_export_png.js">
        </script>
      </xpath>
    </template>

  </data>
</openerp>