checklist of stat and geom layers to adapt to biplots

[corybrunson]

This issue will collect stat, geom, or other layers implemented in other packages (noted unless {ggplot2} itself) that should be adapted to ggbiplot() layers. Cognominal stats and geoms are listed with the stats. See #61 for the list of needed but not-yet-implemented layers.

Statistical transformations (stats):

• StatBin2d, StatBinHex
• StatDensity2d
• StatPointdensity from {ggpointdensity}
• StatHdr, StatHdrLines, StatHdrPoints, StatHdrRug from {ggdensity}
• StatVoronoi from {ggvoronoi}

Geometric elements (geoms):

• GeomPointTrace and GeomPathTrace from {ggtrace}

Layers for 1-dimensional biplots:

• PositionStack (stat = "align") for stacking variable axes or vectors
• StatDensity, GeomDotplot, GeomHistogram, GeomRug