Adding multiple data sets together

Hi,
I'm trying to find a way to add two data sets(contained in different files) together and plot the result.  Is there any easy way of doing this.  For example, is there any way of creating a function, say f(x), which contains your data set?  If that is possible, then I could just read my data files into two functions, f(x) and g(x),  and then plot f(x)+g(x)

Thanks for any help
Chris