Y minimum for auto scale was accidentaly set to 0.
This avoided correct calculation of Y auto log scale (tdf#96807)
But also set linear Y auto scale from 0 (tdf#85690)

This fix detects if both min and max are equal to 0, and reset them (+-inf)

Change-Id: Ifaf306831f3b5a18a86483c88e807f478b447c77
Reviewed-on: https://gerrit.libreoffice.org/21055Reviewed-by: jan iversen <jani@documentfoundation.org>
Tested-by: jan iversen <jani@documentfoundation.org>
Tested-by: Jenkins <ci@libreoffice.org>