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

David Winsemius dwinsemius at comcast.net
Wed Jun 30 22:58:45 CEST 2010

On Jun 30, 2010, at 4:02 PM, Asha Sharma wrote:

> 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?

I suspect you are requesting a scatterplot with a superimposed linear  
fit and 5% and 95% prediction intervals. Not much point in pursuing  
until you "confirm or deny".

> 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".
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David Winsemius, MD
West Hartford, CT

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