Created attachment 89480 [details]
Two charts demonstrating the problem

When charting functions, a cubic spline curve gives counter-intuitive results. In the attached, the first curve would make a reasonable person wonder, who are the outliers? (There are none, all zeros) In the second curve, there seems to be a lot of excess wobbling - you would expect Jan-Mar to be straight, and the line to not go higher than April, but the line peaks /before/ April.

This seems to be caused by the Cubic Spline function treating the X and Y axises as equal. It would be nice if there was a new "functional interpolation" line, smoothing that was geared more for functions.

This is discussed a little here (http://ask.libreoffice.org/en/question/24872/custom-cubic-spline-smoothing-equation-for-line-in-chart/).

Steps to reproduce:
1. Chart a smooth-line, line chart of the points 0,0,0,0,0,20000,0 (see attachment)

Current behavior:
wobbles a lot, apogee of curve is AFTER the 20000 point, the line goes (a little) higher than the highest point, 20000.


From Regina on (http://ask.libreoffice.org/en/question/24872/custom-cubic-spline-smoothing-equation-for-line-in-chart/) "
You are likely looking for a functional interpolation, where the points, which are added by the interpolating algorithm, behave the same as if they were true functions points, and where y depends on x. Such interpolation does not exist in LO. But I agree, that such kind of interpolation is needed. There already exists suitable code in OOo1.1.5 using splines, but other ways are perhaps better. If LO has an implementation it will be possible to get it later on into ODF standard.

              
Operating System: All
Version: 4.1.3.2 release