JpGraph is an OO graph drawing library for PHP 5.1 and above. Highlights of the available features are: multiple scale types; anti-aliasing of lines; color-gradient fills; support for GIF, JPEG, and PNG formats; and support for multiple Y axes. Many plot types are supported, such as pie-charts, lineplots, bar plots, impulse plots, polar plots, error plots, contour plots, matrix plots, gantt charts, and many more. The pro-version also contains extensive 1D and 2D barcode support. The documentation is extensive and is over 700 pages.
tgif is a vector-based drawing tool, with the additional benefit of being sort of a web-browser. That is, you can fetch drawings from a web server with it, and you can make objects in your picture into hotlinks to other parts of the drawing, or to other drawings accessible via HTTP.