The plotannotate package (#RStats) is now officially on CRAN:

https://cran.r-project.org/package=plotannotate

The package allows users to interactively annotate 'base R graphics' plots with freehand drawing, various symbols (points, lines, arrows, rectangles, circles, ellipses), and text. This is useful for teaching, for example to visually explain certain plot elements, and creating quick sketches.