shading for persistence diagram

[rrrlw]

could we consider adding the option to shade the bottom-right triangular half of persistence diagrams? (example image above)

[corybrunson]

I could see how this could be used to distinguish ordinary persistence diagrams from extended persistence diagrams, if both are used in the same project, or just as a cue to the viewer to ignore the southeast triangle. (I don't know how common it is, but it seems natural enough to want.) It should be doable using GeomPolygon internally, though it might be tricky to ensure that the legs of the triangle line up exactly with the plot window.

[corybrunson]

I suggest making this an option of geom_diagonal().