I want to figure out how to make lines etc, but the documentation is very poor, I cannot work out what the arguments to various objects are without trial and error.
Yes, jmathplot is not very well documented, but next version (3.0) will correct this problem.
What exactly do you try to do ?
draw a plot with lines (called line-plot)
or add lines to your plots (just for design)?
When you will planning to release jmathplot 3.0 ?