Here's a write-up I made a couple years ago which I think does a bit better describing the math in question: https://observablehq.com/@jrus/cubic-spline https:

Here's a write-up I made a couple years ago which I think does a bit better describing the math in question: https://observablehq.com/@jrus/cubic-spline https://observablehq.com/@jrus/cubic-spline