|add-to-plot||thing &rest other-things|
|set-title||label &optional (fig nil)|
|set-xlabel||label &optional (fig nil)|
|set-ylabel||label &optional (fig nil)|
Describes how thing has to be plotted. Available for external classes. For example if thing is of class bidule, and has a slot called data that is to be used for plotting, the corresponding method should return something like (list (data bidule instance) :label "bidule-instance").