The easiest-to-find method for shading under a normal density is to use the polygon() command. That link is to the first hit on Google for "Shading Under a Normal Curve in R." It works (like a charm), but it is not the most intuitive way to let users p...

Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).