If you're a mathematician or are involved in statistical analysis, you'll need someway to plot and draw graphs accurately.
Graphviz is a free open source automated graph layout software with a Graphical User Interface to make things as easy as possible. Most importantly, Graphviz supports DOT files and can also export to PDF and several other bitmap formats. For those in the know, it also includes full alpha transparency, native fonts, shapefile support and anti-aliasing.
Graphviz is quite a complex program despite the GUI although the developers assure that the engine behind it - a simple file format known as DOT - intelligently connects nodes to other nodes. A sophisticated layout routine then renders it into many different output formats including SGI, Photoshop, Windows BMP, JPEG, Macintosh PICT, PNG, MacPaint, TIFF, Targa, JPEG 2000 and Quicktime Image. You can preview graphs and if you're struggling with a setting, a "Tooltips" tool automatically assists you.
Graphviz is purely for those that know what they're doing with DOT files and need to plot complex graphs.