[R] Different behavior of mtext

[Prof Brian Ripley]

On Mon, 3 Sep 2007, Sébastien wrote:

> Ok, the problem is clear now. I did not get that 'user-coordinates' was 
> refering to par("usr"), when I read the help of mtext. If I may ask you some 
> additional questions:
> - you mentioned a missing unit() call ; at which point should it be done in 
> my code examples ?

Before it is used.  The problem is that I believe more than one package 
has a unit() function.

> - could you give me some advices or helpful links about how to set up a grid 
> viewport ? - and finally, probably a stupid question: is a gridview 
> automatically set up when a plotting function is called ?

If you want to mix grid and base graphics, you need package gridBase, but 
really I would not advise a beginner to be using grid directly (that is, 
not via lattice to ggplot*).


> Sebastien
>
> PS: To answer to your final question, my goal is to center a block of legend 
> text on the device but to align the text to the left of this block.
>
> Prof Brian Ripley a écrit :
>> On Sun, 2 Sep 2007, Sébastien wrote:
>> 
>>> Dear R Users,
>>> 
>>> I am quite surprised to see that mtext gives different results when it
>>> is used with 'pairs' and with "plot'. In the two following codes, it
>>> seems that the 'at' argument in mtext doesn't consider the same unit 
>>> system.
>> 
>> It is stated to be in 'user coordinates'.  Your code does not work because 
>> unit() is missing.  If you mean the one from package grid, "npc" is not 
>> user coordinates (and refers to a grid viewport which you have not set up 
>> and coincidentally is the same as the initial user coordinate system to 
>> which pairs() has reverted).
>> 
>> Try par("usr") after your pairs() and plot() calls to see the difference.
>> Plotting a 2x2 array of plots _is_ different from plotting one, so this 
>> should be as expected.
>> 
>> Since centring is the default for 'adj', it is unclear what you are trying 
>> to achieve here.
>> 
>>> I would appreciate your comments on this issue.
>>> 
>>> Sebastien
>>> 
>>> ##### Pairs
>>> 
>>> mydata<-data.frame(x=1:10,y=1:10)
>>> 
>>> par(cex.main=1, cex.axis=1, cex.lab=1, lwd=1,
>>>    mar=c(5 + 5,4,4,2)+0.1)
>>> 
>>> pairs(mydata,oma=c(5 + 5,4,4,2))
>>> 
>>> mylegend<-c("mylegend A","mylegend B","mylegend C","mylegend test")
>>> mylegend.width = strwidth(mylegend[which.max(nchar(mylegend))], "figure")
>>> 
>>> for (i in 1:4) {
>>> mtext(text=mylegend[i],
>>>        side = 1,
>>>        line = 3+i,
>>>        at = unit((1-mylegend.width)/2,"npc"),            # centers the
>>> legend at the bottom
>>>        adj=0,
>>>        padj=0)}
>>> 
>>> ##### plot
>>> 
>>> mydata<-data.frame(x=1:10,y=1:10)
>>> 
>>> par(cex.main=1, cex.axis=1, cex.lab=1, lwd=1,
>>>    mar=c(5 + 5,4,4,2)+0.1)
>>> 
>>> plot(mydata,oma=c(5 + 5,4,4,2))
>>> 
>>> mylegend<-c("mylegend A","mylegend B","mylegend C","mylegend test")
>>> mylegend.width = strwidth(mylegend[which.max(nchar(mylegend))], "figure")
>>> 
>>> for (i in 1:4) {
>>> mtext(text=mylegend[i],
>>>        side = 1,
>>>        line = 3+i,
>>>        at = unit((1-mylegend.width)/2,"npc"),            # should
>>> center the legend at the bottom but doesn't do it !
>>>        adj=0,
>>>        padj=0)}
>> 
>

-- 
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