I have used the GD library for simple things like bar charts, pie charts, etc. It works great when you want to read data (from say, a database) and generate a simple plot or chart on-the-fly for a web page. It can create images in .gif, .png, or .jpg formats. I usually prefer .gif. There was a period of time when support for .gif was dropped due to copyright issues, but that is no longer the case. Just make sure you have the latest version installed.