Implement B-Splines interpolation.
Submitted by Vladimir Chukharev
Link to original bug (#338626)
Description
More XY chart subtypes are needed, at least similar to Stacked area (both for linear and spline interpolations) and Stacked columns (with 3 alignments of the columns). More general splines are also needed, for smoothing the curve.
I want to repeat: Thank you for a great program!