[R] how to make lines() meet axis in autoscaled coordinate system?

[Johannes Graumann]

Hello,

I have an auto-scaled coordinate system and would like to add some
clarifying lines to the graph - which ares supposed to meet the axis.

>lines(c(0,time1,time1), c(1,1,0),lty=3)

does what I want, BUT the second leg does not touch the x-axis since the
auto-scaling of the y-axis does not start at'0' but slightly negative.
I could now adjust the line length by 'trial and error' to suit my
needs, but I'd prefer a generalizable solution like

>lines(c(<some_macro_for_y-axis_position>,time1,time1),
+	c(1,1,<some_macro_for_x-axis_position>),lty=3)

Can somebody help me out with this?

Thanks,

Joh