[codemac]

It's not that you want to provide "every possible way to draw a line".. ideally your API provides the primitives necessary for a client to draw lines in all the ways that are useful.

The primitives that you provide relate to the overall goal of your API. Do you want people to use it for mathematical graphing? Will they start with an equation? Or is this for a website, where all you really want is <hr/>.