prevent inconsistency in diagram type across layers

[corybrunson]

A major problem with the diagram parameter is that different values can be applied to different layers, which results in nonsense. Instead, new coordinate system ggproto classes should be used, perhaps CoordFlat and CoordLandscape, which would transform the entire plot.

[corybrunson]

This turned out to be a naïve idea! The different diagrams belong to the statistical transformation; a coordinate ggproto would totally reformat the plot.

The problem of consistency in the use of diagram remains, which might be resolved by making it a settable option.