Inset graphical objects — geom_grob

geom_grob and geom_grob_npc add Grobs as insets to the ggplot using syntax similar to that of geom_text, geom_text_s and geom_text_npc. In most respects they behave as any other ggplot geometry: they add a layer containing one or more grobs and grouping and faceting works as usual. The most common use of geom_grob is to add data labels that are graphical objects rather than text. geom_grob_npc is used to add grobs as annotations to plots, but contrary to layer function annotate(), geom_grob_npc is data driven and respects grouping and facets, thus plot insets can differ among panels. Of these two geoms only geom_grob supports the plotting of segments, as geom_grob_npc uses a coordinate system that is unrelated to data units and data.