What’s it for?
A graph only exists to make a point. Its purpose is not to present all the information. Its purpose is not to be pretty.
Most of all, its purpose is not, "well, they told me I needed to put a graph here."
The purpose of a graph is to get someone to say "a-ha" and to see something the way you do.
Begin there and work backwards.
[Only slightly related: I’ll be in Orange County for an evening event on February 15. Details are here. Hope to see you there.]