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

[Asha Sharma]

Hi,

Thanks to both of you for taking the time to answer my question. I was 
maybe not very clear in the way I framed my question. By plot, I meant 
an x-y plot with a cloud of points which should have a linear lower 
envelope. Is there a way to both plot as well as get the parameters of 
the lower envelope (intercept, slope, etc.) and to also set the 
percentage of points outside it?

Thanks!

Asha


Stephan Kolassa wrote:
> Hi,
>
> one possibility would be to calculate the convex hull using chull(). I 
> believe that the hull points are returned by chull() in a clockwise 
> order (?), so the points between the rightmost and the leftmost point 
> in the chull() result are the lower half of the convex hull. Remove 
> these points from the original dataset (a variant of "peeling convex 
> hulls") and iterate until you have removed your prespecified 
> percentage of points - these all lie outside of the final lower hull 
> (though the percentage will, of course, only be approximated, but you 
> should be able to modify this to taste).
>
> HTH
> Stephan
>
>
> David Winsemius schrieb:
>>
>> On Jun 30, 2010, at 2:05 PM, Asha Sharma wrote:
>>
>>> 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.
>>
>> You probably want something like the "lower half" of the convex hull. 
>> You should find quite a bit of code with your favorite r search 
>> engine on the topic of "convex hull".
>