Note that the PlotBoilerplate already has a Grid instance member. The Grid is not meant
to be added to the canvas as a drawable as it encapsulates more an abstract concept of the canvas
rather than a drawable object.
The VertexAttr is a helper class to wrap together additional attributes
to vertices that do not belong to the 'standard canonical' vertex implementation.
This is some sort of 'userData' object, but the constructor uses a global model
to obtain a (configurable) default attribute set to all instances.