[R] putting circles/buffers arround points on a 2D graph/plot

[Ahmed, Sadia]

Hi, 

I'll try to keep my question brief and to the point, to anyone who helps 'THANK YOU!'

I want to get a circle/ring/buffer or some other form of drawn line around certain points on a x,y plot, the points usually don't form a circle, often they form a 'wobbly' shape, so ideally I'd like something that follows the shape the points make.

I'll start by explaining what I've got (I'm sorry if it's a little long, but it's all important for explaining what I want to do):

I have a simple 2D plot of an x variable, called 'percent.forest'', and a y variable called 'metric'. the data for this plot is in a large data frame with a third variable called 'Buffer', there are 5 buffer sizes (250, 500, 1000, 3000, 10000).

to create the plot I simply used 
>plot(percent.forest,metric)

to add in the data about buffer size, I have extracted the percent forest and metric data by:
>buff250<-subset(data1,data1$Buffer==250)
>buff500<-subset(data1,data1$Buffer==500)
>buff1000<-subset(data1,data1$Buffer==1000)
>buff3000<-subset(data1,data1$Buffer==3000)
>buff10000<-subset(data1,data1$Buffer==10000)

i then used the 'points' function to plot the percent forest, metric data points in different colours according to buffer size, using this code (first 2 lines divides the data appropriately, 3rd line is points):
>metric.250<-(log(buff250$metric))
>percent.forest.250<-(buff250$percent.forest)
>points(percent.forest.250,metric.250,col='skyblue')

I repeated this for all 5 buffer sizes, so on the plot (percent.forest,metric) the points are coloured differently according to the size of the buffer used.
This works fine (the next bit is the problem):

Does anyone know how I can get R to draw around the different coloured points, so that i get 5 rings/wiggly lines on the plot that clearly shows the extent of each of the buffers, you can sort of see a pattern in the colours but its not as clear as it would be if I could draw around it). 

If anyone wants to see a picture of the plot (if my explanation isn't clear enough) I'd be happy to email one directly to you (the R instructions says that the mailing list scraps attachments so I've not put one on here)

Thank you for reading this, 
Any help will be greatly appreciated,
sadia