With a negative intercept, Y values can be negative

Rebase with forced intercept fec037e6

Conflicts:
	chart2/source/tools/ExponentialRegressionCurveCalculator.cxx

Change-Id: Ie351c006fb1688ef3e657da7ce0789a9da1317f0
Reviewed-on: https://gerrit.libreoffice.org/15353Reviewed-by: Philippe Jung <phil.jung@free.fr>
Tested-by: Philippe Jung <phil.jung@free.fr>