How can I make linear and cubic spline interpolations by using formulas? Libreoffice can *draw* interpolated lines using both of these interpolations if I make a graph, but how can I make the interpolation directly into a table?

So far I haven’t been able to locate any plugins of funtions that can solve this.

**Example, linear:**

x1=10, x2=20, y1=30, y2=50

CALCULATE: y for x=15

**Example, spline:**

x1=10, x2=20, x3=30, y1=100, y2=400, y3=900

CALCULATE: y for x=13

There is a plugin for Microsoft Excel that does the trick here, but thats no good for Libreoffice.

Linear interpolation is used all over, and cubic spline is state of the art interpolation used in stuff as engineering an rocket science