Rethink mappingLogicalToScene in XYCurve::retransform(). Performance increase of about factor 2

Maybe instead of using all those points, we can use all points from m_lines (there are less) but the the number of logical and the number of scene points do not match anymore and this mapping must be called after updateLines()

TODO:

• check where m_lines is used
• for nonlinear ranges, already in the addline() function a mapping is done, so we do not need it a second time