What kind interpolation function need be used?

HI

What kind of interpolation function can be used for the data looks like in the graph.

I am using the interp1 function, some one said that I cant use it becuase the data is not monotonic.

Is there any function to deal with these kind of data?

my code is

A=xlsread('interpolation.xlsx','sheet1','A:A')
B=xlsread('interpolation.xlsx','sheet1','B:B')

x=0.2:0.1:0.5
   D=interp1(B,A,x)

Thanks in advance..

[Chris McComb]

What is your motivation for doing the interpolation? Also, can you tell anything about the nature of the data?

[John D'Errico]

What does it mean to interpolate a function that has multiple values for a given value of x?

While there are ways one can interpolate such non-monotonic curves, it does not appear that you know what you want to do here. In fact, I've written such a tool. But first, we need you to understand what you are doing, or we need to understand what it is you want.

One possibility for example, is to just smooth the curve you have.

[R7 DR]

@Chris and John Thanks for the reply

I have a set of Xand Y values and I need to find the X value at known value of Y. For example, I want to find the value of X at Y=0.2,0.36, 0.42.....etc. As,the data is not in a good shape, some times I am getting negative values while doing the interpolation.

Do we have any function for curve smoothing? Does the smoothing of the curve changes the data??