[R] Finding Lower Envelope of Points on a Plot?

[Asha Sharma]

Hi,

I am looking for a way to find the lower envelope of points on a plot, 
preferably specifying what percentage of points should be allowed to lie 
outside the envelope. There must be a straightforward way to do this, 
but I do not seem to be able to find it. I would greatly appreciate any 
help.

Thanks for your time,

Asha