PlotFormat is a general category which will include many subtypes, such as PointPlot, BarPlot, MeshPlot, etc. PlotFormats are attached to the graph in an order that corresponds to the order of the Datasets applied to the same graph. If there are fewer PlotFormats than Datasets, the PlotFormats will be cycled through as needed. Because PlotFormats and Datasets are attached separately, it is easy to change the format of a graph without changing the data, or vice versa.
A PlotFormat works by taking a Dataset given to it by the Graph, and returning a set of 3D drawing primitives. These are then combined with all the other primitives and rendered. To extend Graphite's capabilities with a new plot type, it should suffice to create a new subclass of PlotFormat; all details of data handling, rendering, and so on will be handled by other classes.