The graphviz package is used by the TGraphStruct class to represent structural
information as diagrams of abstract graphs and networks.

This package can be downloaded from http://www.graphviz.org/.

To find graphviz the ROOT's configure file looks in standard locations. It is
possible to define a specific location using the configure flags:

--with-gviz-incdir="the directory where gvc.h is"
--with-gviz-libdir="the directory where the libgvc library is"

To install graphviz it is recommended to use the following configure flags:

--enable-static=yes --enable-shared=no --with-pic --prefix="graphviz installed here"

On 64 bits machines, the ROOT sources are compiled with the option -m64.
In that case graphviz should be also compiled in 64 bits mode. It might be the
default option, but on some machine it is not. In that case the environment
variable CC should be defined as:

CC="gcc -m64"

before doing configure.

On Windows machines it recommended to not install graphviz but to download the
pre-installed version from http://www.graphviz.org/. The ROOT configure command
remains the same.