[R] Identifying points in a plot that have duplicate values

Chuck Cleland ccleland at optonline.net
Mon Mar 5 16:29:05 CET 2007


David Lloyd wrote:
> I have code like this: - 
> 
> #-----------------------------------------------------------------------
> ------------------------------------------------------
> 
> x=scan()
> 0 0 0 0 0 1 2 3 4
> 
> y=scan()
> 1 1 1 2 2 1 3 4 5
> 
> plot(x,y)
> 
> identify(0,1,3) #Allows me to select manually to identify co-ordinate
> (0,1) as being duplicated 3 times
> identify(0,2,2) #Allows me to select manually to identify co-ordinate
> (0,2) as being duplicated 2 times
> #-----------------------------------------------------------------------
> ------------------------------------------------------
> 
> Is there not a way I can automatically display if points are duplicated
> and by how many times?
> 
> I thought if I 'jittered' the points ever so slightly I could get an
> idea of how many duplicates there are but with >100 points the graph
> looks very messy.

  You might consider using alpha transparency - the more times a point
is duplicated the darker it will be.  For example:

df <- data.frame(x=c(0, 0, 0, 0, 0, 1, 2, 3, 4),
                 y=c(1, 1, 1, 2, 2, 1, 3, 4, 5))

pdf("alphaExample.pdf", version = "1.4", width = 6, height = 6)

with(df, plot(x,y, col=rgb(1,0,0,.3), pch=16))

dev.off()

RSiteSearch("alpha transparency")

> Regards
> DaveL
> 
> 
> 
> 
> 
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-- 
Chuck Cleland, Ph.D.
NDRI, Inc.
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