[R] line over multiplot

[Thomas Hoffmann]

Dear Listmembers,

I do have a multiplot with 1 row and 4 columns (4 scatter plots with 
same x- and y-axe plotted in one row). I would like to draw a horizontal 
line across the whole multiplot at a certain y-value. Unfortunately the 
normal abline command stops between the plot.

I hope my problem is clear and there is somebody who can help me

Thanks in advance
Thomas H.