std::sort is unstable sort, which is not what we want.  Use std::stable_sort
to sort data points by X values.  If we use unstable sort, it may mess up
the order of the sequence when two data points contain identical X values.

Change-Id: I6453a986185b326dc680fbcec6227ea332235b22
Reviewed-on: https://gerrit.libreoffice.org/1871Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>