[areop]

The right answer for a API to draw a line like you describe is to provide every possible way to draw a line since you never know which one the user wants to use. Granularity is one of the general five properties of every good API, of course it is not always good to provide all possible ways since it hurts compactness. The best I have ever heared about API design was a talk from Casey Muratori: http://mollyrocket.com/casey/stream_0028.html

[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/>.