cl-2d
Cl-2d is a two-dimensional graphics library for Common Lisp. It builds on cl-cairo2 and thus uses the Cairo library, which provides various backends, including pixel-based (eg PNG, X11) and vector-based (PDF, SVG, PostScript) ones. It is meant solely for 2-dimensional plotting (hence the name). Cl-2d is available from Github or via ASDF-install. It was originally licensed under the LLGPL but recently (2012-01) changed to the simple, permissive non-copyleft free Boost Software License.

Some example plots can be found on Tamas K Papp's blog.

ASDF-install package (obsolete) http://github.com/tpapp/cl-2d/tarball/master