[R] legend for several graphics

[Greg Snow]

I think you need a par(xpd=NA) before the legend command (At least it
did not show up for me until I set xpd).

For positioning the overall title and legend, one option is the
cnvrt.coords function in the TeachingDemos package.  

To center the overall title over the central column plots (the default
centers it left to right in the window, but with the margins of the
plots this looks a little off): after plotting one of the 3 center
plots, call cnvrt.coords with an x value of 0.5 in plot coordinates
(input='plt') and save the x value of the "tdev" coordinates.  Then use
this value as the at argument to mtext.

To find the values for positioning the legend, give the function the
coordinates in terms of the device (input='tdev', x can either be .5 for
centered or the value computed for mtext above) and store the user
coordinates to pass to legend.

My modification of your example is:

library(TeachingDemos)
op <- par(mfrow = c(3,3), ## split region
          oma = c(5,0,4,0) + 0.1, ## create outer margin
          mar = c(5,4,2,2) + 0.1) ## shrink some margins 
plot(1:10, main = "a", pch = 1:2, col= 1:2) 
plot(1:10, main = "b", pch = 1:2, col= 1:2) 
tmp1 <- cnvrt.coords( 0.5, 0, input='plt' )$tdev # save location for
mtext
plot(1:10, main = "c", pch = 1:2, col= 1:2) 
plot(1:10, main = "d", pch = 1:2, col= 1:2) 
plot(1:10, main = "e", pch = 1:2, col= 1:2) 
plot(1:10, main = "f", pch = 1:2, col= 1:2) 
plot(1:10, main = "g", pch = 1:2, col= 1:2) 
plot(1:10, main = "h", pch = 1:2, col= 1:2) 
plot(1:10, main = "i", pch = 1:2, col= 1:2) 
## title 
mtext("My Plots", side = 3, outer = TRUE, font = 2, line = 1, cex = 1.2,

	at=tmp1$x) 
## draw legend 
par(xpd=NA)
tmp2 <- cnvrt.coords( tmp1$x, 0.05, input='tdev' )$usr # get location
for legend
legend(tmp2$x, tmp2$y, legend = c("Type 1", "Type 2"), 
 	pch = 1:2, col = 1:2, ncol = 2, xjust=0.5, yjust=0.5)
par(op)


Hope this helps, 


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

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Gavin Simpson
> Sent: Wednesday, March 05, 2008 11:43 AM
> To: Georg Otto
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] legend for several graphics
> 
> On Wed, 2008-03-05 at 15:28 +0100, Georg Otto wrote:
> > Hi,
> > 
> > I am trying to generate a figure of 9 plots that are 
> contained in one 
> > device by using
> > 
> > par(mfrow = c(3,3,))
> > 
> > I would like to have 1 common legend for all 9 plots 
> somewhere outside 
> > of the plotting area (as opposed to one legend inside each of the 9 
> > plots, which the function legend() seems to generate by default).
> > 
> > Any hint how to do this?
> 
> Here's one way:
> 
> op <- par(mfrow = c(3,3), ## split region
>           oma = c(5,0,4,0) + 0.1, ## create outer margin
>           mar = c(5,4,2,2) + 0.1) ## shrink some margins 
> plot(1:10, main = "a", pch = 1:2, col= 1:2) plot(1:10, main = 
> "b", pch = 1:2, col= 1:2) plot(1:10, main = "c", pch = 1:2, 
> col= 1:2) plot(1:10, main = "d", pch = 1:2, col= 1:2) 
> plot(1:10, main = "e", pch = 1:2, col= 1:2) plot(1:10, main = 
> "f", pch = 1:2, col= 1:2) plot(1:10, main = "g", pch = 1:2, 
> col= 1:2) plot(1:10, main = "h", pch = 1:2, col= 1:2) 
> plot(1:10, main = "i", pch = 1:2, col= 1:2) ## title 
> mtext("My Plots", side = 3, outer = TRUE, font = 2, line = 1, 
> cex = 1.2) ## draw legend legend(-12.5, -6, legend = c("Type 
> 1", "Type 2"), pch = 1:2, col = 1:2, ncol = 2)
> par(op)
> 
> I had to fiddle by hand with the legend x and y locations to 
> get it roughly centred. There has to be better way - probably 
> something to do with reseting the plot region, but I can't 
> recall how to do that now. If there is, I'm sure someone will 
> tell me what I overlooked.
> 
> Is this what you were looking for?
> 
> G
> 
> > 
> > Best,
> > 
> > Georg
> > 
> > ______________________________________________
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