Remove visual noise from chart2

Change-Id: Ic8dc71c2815df265e1935e629e0220396a50aaac
Reviewed-on: https://gerrit.libreoffice.org/8239
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>