The NaN value forces the scaling of the axis to be based on years
and introduces gaps in the rendering.

Change-Id: I78219be289d76edb53b5672209e1c031ab62def9
Reviewed-on: https://gerrit.libreoffice.org/64267
Tested-by: Jenkins
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

related tdf#42915, pass NaN through date transformation

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>
Reviewed-on: https://gerrit.libreoffice.org/64304
(cherry picked from commit 55c53825)
Reviewed-on: https://gerrit.libreoffice.org/64310Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>