ggplot2: geom_text() with facet_grid()?

Question

I just want to add annotation to each panel of figures generated by ggplot2; just simple labels like (a), (b), (c), etc. in each corner. Is there a simple way to do this?

Answer 1

Basically, you create a data.frame with the text which contains a column with the text, and a column with the variables you use for facet_grid. You can then simply add a geom_text with that data.frame. See the documentation of geom_text for more details on text placement and such.