The graphical state of created windows is stored as structures, which can be handled through the commands documented in this section. The operations change the state of the specified graphic structure. The graphical subsystem contains two representation levels. The higher level defines graphical objects by their meanings such as bar, data scatterplot and so on. A lower level representation is generated from the higher level objects when a window needs redrawing. The lower level contains only really primitive graphical elements (line, circle, polygon, text). Therefore, the lower level representation can easily be displayed using any graphical user interface with a straightforward technical implementation.