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

Asha Sharma ans62 at cornell.edu
Wed Jun 30 22:02:24 CEST 2010


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?



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).
> 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".

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