[R] Very inaccurate circles

[Rolf Turner]

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

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