What is the mathematical function/formula to convert line into spline

line general eq is y=mx+c how can we use it to generate spline (to smooth the line)

How is this related to Libreoffice?

Personally, I think there is a bug (sort of) in the spline equation. I can only guess at why the question was asked.

I’m not an expert, but…

A spline is a “spliced line” - the spliced part, I believe, refers to the equation. Each two points have a different custom equation between them - its a splice of multiple equations with multiple individual formulas.

There is a heap of detail in ODF v1.2 specification, Part 1, under 20.26 chart:interpolation that indicates the formulas used. Cubic splines are detailed pp.693-694. This is not “y=mx+b” stuff - these are formulas to create formulas.

This discussion may also be of interest to you too.

Good Luck! Is this what you were after?

If your question is related to LibO I can imagine that you want to create a smooth line chart.
If so, you can use the build in smooth function. See Chart type / Line chart and change “straight” to “smooth”.
However I don’t know the algorithm behind the smoothing function in LibO.

You have already a straight line. How much more do you want it to “smooth”?