[R] Placing a Shaded Box on a Plot

[Stephanie Cooke]

Hello,

I would like to place shaded boxes on different areas of a
phylogenetic tree plot. Since I can not determine how to find axes on
the phylogenetic tree plot I am not able to place the box over certain
areas. Below is example code for the shaded box that I have tried to
use, and the first four values specify the position.

rect(110, 400, 135, 450, col="grey", border="transparent")

Any suggestions on how to place a shaded box to highlight certain
areas of a plot I would greatly appreciate.