 Solution

In model-view-controller, the model owns application data and its rules: here it is the ordered DataList with its uniqueness condition on horizontal coordinates. A view renders those data, for example as a line plot or a bar plot. View-specific decisions such as whether to show axes need not change the model. A controller interprets actions such as entering a new sample or selecting a plot style, and requests the appropriate model or view operation.

This separation of concerns permits several views of the same data and allows model operations to be tested without drawing a window. When the model changes, affected views need a notification or another refresh mechanism. The Observer pattern in part (d) supplies one such mechanism, without making the model depend on every concrete view software class. The division is