Generic Mapping Tools (GMT) is an open source collection of ~60 tools for
manipulating geographic and Cartesian data sets (including filtering,
trend fitting, gridding, projecting, etc.) and producing Encapsulated
PostScript File (EPS) illustrations ranging from simple x-y plots via
contour maps to artificially illuminated surfaces and 3-D perspective
views. GMT supports ~30 map projections and transformations and comes
with support data such as GSHHS coastlines, rivers, and political
boundaries. GMT is developed and maintained by Paul Wessel and Walter
H. F. Smith with help from a global set of volunteers, and is supported
by the National Science Foundation. It is released under the GNU General
Public License.

GMT requires netcdf, available at SlackBuilds.org.

In order to build GMT, there are a number of required archives:

GMT4.5.1_doc.tar.bz2 
GMT4.5.1_share.tar.bz2 
GMT4.5.1_src.tar.bz2
GMT4.5.1_suppl.tar.bz2 
GMT4.5.1_triangle.tar.bz2 (if you require Shewchuk's triangulation 
routine (see below)).

These must all be downloaded and placed in the same directory as the
SlackBuild script (as with any source).

There are a number of different resolution coastline files available
for GMT, and their size on disk varies from <2MB (low) to 61.7MB
(full). They come in 3 separate archives:

GSHHS2.0.1_coast.tar.bz2 
GSHHS2.0.1_high.tar.bz2 
GSHHS2.0.1_full.tar.bz2

The first contains the low, intermediate and coarse resolutions,
the second the high resolution, and the third the full resolution. I
prefer to have all of them installed (disk space is cheap), but you
may not. Therefore, by default, all three sets will be installed,
unless you choose otherwise in the script. e.g. ${COASTS:-"coast"}
would install only the low, intermediate and coarse resolutions. A
minimum of GSHHS2.0.1_coast.tar.bz2 is required.

To download all the files, the following works: 
wget \
  ftp://ftp.soest.hawaii.edu/gmt/GMT4.5.1_{doc,share,src,suppl,triangle}.tar.bz2 \
  ftp://ftp.soest.hawaii.edu/gmt/GSHHS2.0.1_{full,coast,high}.tar.bz2

Please note, Shewchuk's triangulation routine used in some parts of
GMT is not distributed under the GNU Public Licence. If you work for a
for-profit organisation, you should read Shewchuk's copyright statement
(in $TMP/$PRGNAM-$VERSION/src/README.TRIANGLE) first. This script enables 
the (much faster) Shewchuk triangulation routine by default; should you 
need to change this behaviour, set SHEWCHUK to "no" in the build script.