If you are creating a new geom, stat, position, or scale in another package,
you'll need to extend from ggplot2::Geom, ggplot2::Stat,
ggplot2::Position, or ggplot2::Scale.
Drawing axis annotations requires overloading the normal
ggplot_build and ggplot_gtable to add extra columns and rows for these
annotations (see gtable_show_grill).
See file axis-annotation.r.
The CoordFlexCartisan and CoordFlexFlipped allows us to inject other
render_axis_h and render_axis_v methods for drawing other axes.
See coord-flex.r.
The class FacetGridRepeatLabels is an extension of ggplot2::FacetGrid
while FacetWrapRepeatLabels is an extension of ggplot2::FacetWrap,
both draws the axes on each panel.
See facet_rep_grid, facet_rep_wrap, facet-rap-lab.r and facet_wrap.r.
The geom-objects here extends ggplot2::Geom
allowing us to produce the geoms for
geom_pointline,
geom_pointpath, geom_pointrangeline, and geom_siderange.
See geom-pointline.r and geom-siderange.r.