Symbolic Integration using fnint

Hello everyone,

I am trying to realize an Abel Inversion of some data. One way to do this is to create a spline-interpolation of the data, differentiate that spline-function and then use the following formula:

http://upload.wikimedia.org/math/5/f/a/5fa206c4c56dd4e321fcd7fd0001cb62.png

I(y) denotes the spline-function. I have tried to do this integral using fnint, but apparently it cannot cope with symbolic expressions, i.e. keeps complaining about the y- and r-symbols.

I would be very thankful for helpful hints.

Best Regards Chris