The implementation of plotting in XSPEC12 is designed to produce a clean separation between the Model and the View layers. A Singleton object (Plot) has the task of gathering data from the problem domain for producing graphical output. It also has the task of making on user request Command objects that implement and sequence user requests for individual plotting modes. Thus the Plot object itself belongs to the Model layer

The actual graphical package (initially PLT/PGPLOT for backward compatibility with XSPEC11--)  has an implementation that is separated from Plot using the Strategy pattern. Plot contains a pointer to Graph which provides an abstract interface for generating plots. PLT/PGPLOT is a concrete instance of graph. Other classes with similar hierarchies implement necessary features such as multipanel layout.