A customer (xfab) asked to have in the same graph several histograms overlaid (with colors) and also the densities overlaid, especially normal densities
I believe it is important for them because some of the trainees have used it in other software
and for me densities overlay are a good way to compare visually
(I know we can do it for non parametric densities and it is nice but several customers also want THE normal densities :)