R help - Sep 2009 - Very inaccurate circles

Hello,

Please take a look at the attached plot and let me know if this is
normal. The circle has radio  I am using R 2.9.2 inside OS X Leopard.

The plot was generated with:

png('bizarre_circle.png')
plot(c(-5,0,0,5), c(0,5,-5,0))
symbols(0,0, circles=c(sqrt(25)), inches=FALSE, add=TRUE)
dev.off()

If I resize the x11's plot window I can manage to make the circle pass
through all the points but it's the first time I have to do this. I
understand this may be happening because of the method to draw
circles. I googled and found this other methods:

http://tolstoy.newcastle.edu.au/R/help/03a/3364.html

But method 2 and method 3 seem to use different units than my points
and working with them is not so straightforward.

1. Is there a way to draw circles in the same unit system than plot
already uses?

2. Is there a way of making method 3 (which uses mm) work with the
same unit system as the plot?

Thank you,

-- 
slnc
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On 8/09/2009, at 11:58 AM, Juan Alonso wrote:
> Hello,
>
> Please take a look at the attached plot and let me know if this is
> normal. The circle has radio  I am using R 2.9.2 inside OS X Leopard.
>
> The plot was generated with:
>
> png('bizarre_circle.png')
> plot(c(-5,0,0,5), c(0,5,-5,0))
> symbols(0,0, circles=c(sqrt(25)), inches=FALSE, add=TRUE)
> dev.off()
	<snip>

Please don't post implied criticism of software (``very
inaccurate'')
when the fault lies not in the software but in your understanding.

There is *nothing* inaccurate about the circles.  The symbols() function
is cleverly designed to produce ``circles'' that ***look*** like circles
when plotted.  In most cases they ***aren't*** circles, but rather  
ellipses,
with eccentricity adjusted to compensate for the aspect ratio of the  
plot
on which they are being superimposed.

If you want the symbols()-created circle to pass through points which  
really
do lie on a circle you need to make the aspect ratio of the plot  
equal to 1:

plot(c(-5,0,0,5), c(0,5,-5,0),asp=1)
symbols(0,0, circles=c(sqrt(25)), inches=FALSE, add=TRUE)

OMMMMMMMMM!

	cheers,

		Rolf Turner

######################################################################
Attention:\ This e-mail message is privileged and confid...{{dropped:9}}

Try this:

library(plotrix)
plot(c(-5,0,0,5), c(0,5,-5,0), asp = 1)
draw.circle(0, 0, 5)


On Mon, Sep 7, 2009 at 7:58 PM, Juan Alonso<s at slnc.me>
wrote:
> Hello,
>
> Please take a look at the attached plot and let me know if this is
> normal. The circle has radio ?I am using R 2.9.2 inside OS X Leopard.
>
> The plot was generated with:
>
> png('bizarre_circle.png')
> plot(c(-5,0,0,5), c(0,5,-5,0))
> symbols(0,0, circles=c(sqrt(25)), inches=FALSE, add=TRUE)
> dev.off()
>
> If I resize the x11's plot window I can manage to make the circle pass
> through all the points but it's the first time I have to do this. I
> understand this may be happening because of the method to draw
> circles. I googled and found this other methods:
>
> http://tolstoy.newcastle.edu.au/R/help/03a/3364.html
>
> But method 2 and method 3 seem to use different units than my points
> and working with them is not so straightforward.
>
> 1. Is there a way to draw circles in the same unit system than plot
> already uses?
>
> 2. Is there a way of making method 3 (which uses mm) work with the
> same unit system as the plot?
>
> Thank you,
>
> --
> slnc
>
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>
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