Bug 53875

Problem description: in a chart, the B-spline option should produce a composite function that does NOT pass through all given data points (that's the business of the cubic spline), but instead goes through them with a smoothed behaviour. That's what it did until 3.4, and it was quite useful. 
Now since some not totally clear remake of it, it interpolates all data points, just as a normal spline, but it should not do this. 
So now the two options "cubic spline" and B-spline" in the chart options give uselessly equivalent results.
I tried to understand the motivations behind the change in 3.5 documentation but failed. 
Anyway the fact is that a very nice option is totally gone, and you cannot recreate it manually, since the points shown on the chart with this options are not computable through any calc function.
What's worse, if you open old .ods files with such charts and correct b-spline showing, they are automatically updated to the new uncorrect interpolating instead of smoothing version.