[R] Question on "coherency" plot

[Reza Seirafi]

Hello everyone,
I would highly appreciate your assistance with the following:
I would like to add a line to the coherency plot of a multivariate time 
series, denoting the critical value above which the null hypothesis of 
having no coherency between the two series may be rejected.
Is there a mechanism for doing so in R? Also, is there away to eliminate the 
confidence intervals of the coherency function.
Thanks,
reza