R help - May 2006 - multiple plots with par mfg

Hi,

I'm trying to add points to 2 plots on the fly using par(mfg=vector)  
so switch between them. However, the appropriate scales aren't  
switched when changing from one plot to another, e.g.

par(mfcol=c(2,1))
plot(1,1, col="blue")        # blue plot
plot(1.2,1.2, col="red")     # red plot
points(1.1,1.1)   # appears to bottom left of red point
par(mfg=c(1,1))   # switch plots
points(1.1,1.1)   # should appear at top of blue point, but appears  
as on red plot
#
 > version
          _
platform powerpc-apple-darwin7.9.0
arch     powerpc
os       darwin7.9.0
system   powerpc, darwin7.9.0
status   Patched
major    2
minor    2.1
year     2006
month    03
day      02
svn rev  37488
language R


Is this a bug? if not, can anyone suggest a way of appending to 2  
separate plots on the fly.

Thanks

Yan Wong
Leeds

On Tue, 23 May 2006, Yan Wong wrote:
> Hi,
>
> I'm trying to add points to 2 plots on the fly using par(mfg=vector)
> so switch between them. However, the appropriate scales aren't
> switched when changing from one plot to another, e.g.
>
> par(mfcol=c(2,1))
> plot(1,1, col="blue")        # blue plot
> plot(1.2,1.2, col="red")     # red plot
> points(1.1,1.1)   # appears to bottom left of red point
> par(mfg=c(1,1))   # switch plots
> points(1.1,1.1)   # should appear at top of blue point, but appears
> as on red plot
> #
> > version
>          _
> platform powerpc-apple-darwin7.9.0
> arch     powerpc
> os       darwin7.9.0
> system   powerpc, darwin7.9.0
> status   Patched
> major    2
> minor    2.1
> year     2006
> month    03
> day      02
> svn rev  37488
> language R
>
>
> Is this a bug? if not, can anyone suggest a way of appending to 2
> separate plots on the fly.
No, it is user error.  par(mfg=) specifies where the next figure will the 
drawn, and points() does not draw a figure but adds to one. As the help 
page says:

      'mfg' A numerical vector of the form 'c(i, j)' where
'i' and 'j'
           indicate which figure in an array of figures is to be drawn
           next (if setting) or is being drawn (if enquiring).
           ^^^^

You need to use screen() or layout() to switch back to an existing plot.
>
> Thanks
>
> Yan Wong
> Leeds
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

The  following works for my quick tests, but being undocumented it is
not guarenteed to work for all situations.

The best thing to do is to create the first plot, add everything to the
first plot that you need to, then go on to the 2nd plot, etc.  If you
really need to go back to the first plot to add things after plotting
the 2nd plot then here are a couple of ideas:

Look at the examples for the cnvrt.coords function in the TeachingDemos
package (my quick test showed they work with layout as well as
par(mfrow=...)).

The other option is when you use par(mfg) to go back to a previous plot
you also need to reset the usr coordinates, for example:
> par(mfrow=c(2,2))
> plot(rnorm(10), rnorm(10)
> tmp <- par('usr')
> hist(rgamma(1000,3)) # changes coordinate system
> par(mfg=c(1,1)) # go back to first plot
> points(0,0, col='red')  # wrong place, based on hist coordinate
system
> par(usr=tmp)  # reset coordinates to correct values
> points(0,0, col='blue')  # now it is in the right place

Hope this helps, 


-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
 

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Yan Wong
Sent: Tuesday, May 23, 2006 6:05 AM
To: R-help
Subject: Re: [R] multiple plots with par mfg


On 23 May 2006, at 12:48, Prof Brian Ripley wrote:
> On Tue, 23 May 2006, Yan Wong wrote:
>> if not, can anyone suggest a way of appending to 2 separate plots on 
>> the fly.
>
> No, it is user error.  par(mfg=) specifies where the next figure will 
> the drawn, and points() does not draw a figure but adds to one. As the
> help page says:
>
>      'mfg' A numerical vector of the form 'c(i, j)' where
'i' and 'j'
>           indicate which figure in an array of figures is to be drawn
>           next (if setting) or is being drawn (if enquiring).
>           ^^^^
OK. I didn't appreciate the distinction between drawing and adding.
> You need to use screen() or layout() to switch back to an existing 
> plot.
Thanks, but the help page for screen says "The behavior associated with
returning to a screen to add to an existing plot is unpredictable and
may result in problems that are not readily visible." I assume this to
mean that I shouldn't do it using screen().

I can't find any description of how to add points to several different
plots generated after a layout() call. Is there a way?

Cheers

Yan

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