Problem with export of chart graphics to PDF or ODT if wiki is not public readable

Graphics generated by the chart macro are not exported PDF or ODT format.

The graphics are generated within the directory /xwiki/bin/temp/space/page/chart/.

My investigation of the problem leads me to the following hypothesis:

If the generated link to the graphic is generated as class “wikiexternallink”, an internal http request to the graphic is generated during the generation of the PDF or ODT file.
It uses the external url of the generated temp-file.

However, this request is not fulfilled because the user of this http-request is not logged in.

The result is only a “broken link”.

If I am correct, the chart macro needs to be adjusted.

Any help is greatly appreciated.