The original design of Unix which made all operations stream based made it rather difficult to organise Graphics. The solution to this was the X-Windows system designed and coordinated by the MIT X-Windows consortium. Almost all Graphics workstations on the market today are X-windows based (some others provide X-Windows compatibility). For many of the basic utilities for the X-Windows system and also for a number of nice new features one need go no further than the usual public doamin archives.
In particular, X-Windows version of the popular editors and debugging tools, viewers for plots and pictures in various formats. Toolkits for designing graphical interfaces and so on can all be found.
A number of tools to use graphics without X-windows are also available. The principal among these is GNUPlot which can plot graphs in a variety of specifications to a variety of different output devices and formats.