Support regression curve name in ODF.

Change-Id: Ic065d63eee507906febfa15eb6d5ae1e7de58280
Reviewed-on: https://gerrit.libreoffice.org/6874
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>