fdo#40315 Change png for moving average trendline

Change-Id: If9c27b090b4a3b314bf61eb4b5ba54c6b6ee1961
Reviewed-on: https://gerrit.libreoffice.org/6728
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>