The curves are convex (usually) to represent the law of averages. A scatter-plot would work for most of the curved graphs, but a curve seems more intuitive.
More information about formatting options
About.com Literature: Classic BC Books Beatrice Booksquare Guardian Unlimited: Arts blog – Books The Elegant Variation Mediabistro.com – GalleyCat Metaxucafe The Millions Paper Cuts (The New York Times Blog) The Tarcher Blog (Tarcher Books) The Penguin Blog UK
The curves are convex
The curves are convex (usually) to represent the law of averages.
A scatter-plot would work for most of the curved graphs, but a curve seems more intuitive.