[R] Adding polygons to a barplot

[Jamieson Cobleigh]

I have a barplot I have created using barplot2 and I have been able to
add points and lines (using the points and lines methods,
respectively).  I now need to add some polygons (triangles in
particular), that I want to be shaded to match bars in the plot.  I
can get the coordinates of the corners of the triangles, but don't
know how to draw the triangles.  I know there is the grid.polygon
method, but I don't know how to get it to draw on my plot.  Any help
would be appreciated.

Thanks!

Jamie