Many places in chart2 use NaN to mean no available value. Not propagating
NaN through the helper disables all this functionality.

Change-Id: I37f966007b5b7cc16778c5c6903710fbd144631b
Reviewed-on: https://gerrit.libreoffice.org/64266
Tested-by: Jenkins
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>