Fix center calculation

Description

calculate the center point based on linear scaling, because the function is used to determine the center of the plot

This fixes also a few other problems as well:

• initial placing worksheetelements (textlabel, custompoint, ...) on nonlinear plot ranged plots.
• Position of the axis ticks on the correct side of the line

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: https://bugs.kde.org/show_bug.cgi?id=477691

When external dependencies are removed

• Reporting to ...