PGPLOT is a library of plotting subroutines written by Tim Pearson, callable from Fortran and C, that have been used in a variety of astronomy programs, and some other software packages. PGPLOT is free for non-commercial use and downloadable as source. Although a new version has not appeared for some time, it is convenient, flexible, and many people and a number of useful programs depend on it.
Historically you typically installed PGPLOT by either compiling from source or installing a software package that included a compiled library (libpgplot.a) and various other stuff like fonts. This used to be fairly easy on Linux or Mac, but starting around OS X 10.6 and/or 64-bit CPU Macs, I and other people had difficulty compiling PGPLOT on OS X. Typically one gets weird errors from ranlib during the build process.
It turns out that this has to do with whether the Fortran and C compilers are defaulting to compile 32 or 64-bit code. The solution is to have new configuration files that properly set PGPLOT up to compile under OS X, with the compiler of your choice (g77 or gfortran) and create either a 32 or 64 bit library.
Updated configuration files and step by step instructions for compiling PGPLOT are now at http://mingus.as.arizona.edu/~bjw/software/pgplot_fix.html
(A tar file of the configuration files is also attached to this wiki page.)
These instructions are experimental and in particular I've only tested them on OS X 10.6, so I welcome feedback on whether they work on OS X 10.7. Email bjw -at- as.arizona.edu.