In reply to Rick Wicklin.

We are comparing the income kernel densities of non-migrants and migrants (prior to migration) and we would like to plot the difference between densities to better show the positive selection (as Jesus Moraga did in: New evidence on emigrant selection, Review of Economics and Statistics, 2011).
In R, for instance, this would be done by computing the arithmetic difference between the linear interpolation of the two kernel density functions. I am still looking for a book/paper that would provide some sort of theoretical background.