Alternatively, you could make use of transparency (on some devices), or use
ggplot2 to map the number of observations to the point size,

d =
read.table(textConnection("
x       y
1       10
1       10
2       3
4       5
9       8
"),head=T)

library(ggplot2)

# transparency
qplot(x, y, data=d, alpha=I(0.5))

d2 = unique(ddply(d,.(x,y), transform, count=length(x)))
# mapping number of obs.
qplot(x, y, data=d2,size=count)

HTH,

baptiste

2009/7/15 Chuck Cleland <ccleland@optonline.net>

> On 7/15/2009 2:19 PM, NDC/jshipman wrote:
> > Hi,
> >     I am new to R plot.  I am trying to increase the data point
> > observation when duplicate data points exist
> >
> > x    y
> > 1    10
> > 1    10
> > 2    3
> > 4    5
> > 9     8
> >
> >
> > in the about example  1, 10 would be displayed larger than the other
> > data points.  Could someone give me some assistance with this problem
>
>   A couple of simple approaches:
>
> x <- c(1,1,2,4,9)
>
> y <- c(10,10,3,5,8)
>
> plot(jitter(x), jitter(y))
>
> plot(x, y, cex=c(2,2,1,1,1))
>
> > 757-864-7114
> > LARC/J.L.Shipman/jshipman
> > Jeffery.L.Shipman@nasa.gov
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> --
> Chuck Cleland, Ph.D.
> NDRI, Inc. (www.ndri.org)
> 71 West 23rd Street, 8th floor
> New York, NY 10010
> tel: (212) 845-4495 (Tu, Th)
> tel: (732) 512-0171 (M, W, F)
> fax: (917) 438-0894
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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