abline for quantmod charts

Hi,

I was trying to add vertical lines to a quantmod plot using

abline( v = somdatetime ) like in the following example

getSymbols("YHOO")

[1] "YHOO"

chartSeries(YHOO)
abline( v=as.POSIXct("2008-04-01","%Y-%m-%d") )

But nothing shows up on the chart.

Any ideas on how to do this?

Thanks
Haky