Abstract: In scientific and engineering computations, after performing calculations, we need to visualize numerical results for understanding and analysis. In doing so, we face the problem of choosing an efficient tool for visualization that would be easy-to-learn and convenient to use. Presently gnuplot27 is probably the most well-known program for plotting and visualizing data. gnuplot is based on a system of commands which can be employed both interactively and in scripts. In this chapter, we consider plotting and preparing data for visualization as well as the processing of results of calculations using gnuplot. We also provide a brief description of the main features of gnuplot.