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.
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.