I used gnuplot a while ago because I didn't want to have to deal with the line-drawing logic GD required. (I also haven't used GD in a couple years.) With gnuplot the script just generats a text file and runs gnuplot.

http://www.gnuplot.info/demo/finance.html has some financial graphing using gnuplot.

There is also flot that does all the graphing in javascript.