Help with a Lagrange interpolating polynomial

Hi all,

I have a very large data set(2 columns of about 3800) in which I plotted in matlab. For this data set, i'm trying to write a code that will bring the graph down to 0 by curve fitting a Lagrange interpolating polynomial to the points between the peaks and subtracting this resulting polynomial from the original set. I want to use values of 10, 20, 35, 60 and 80 for 2theta.

But my problem is that I am unsure how to approach this. Any help?