[R] Specifying relative position of text in a plot

[Prof Brian Ripley]

On Wed, 12 Mar 2008, Nordlund, Dan (DSHS/RDA) wrote:

>> -----Original Message-----
>> From: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r-project.org] On Behalf Of Alberto Monteiro
>> Sent: Wednesday, March 12, 2008 10:28 AM
>> To: Tom La Bone; r-help at r-project.org
>> Subject: Re: [R] Specifying relative position of text in a plot
>>
>>
>> Tom La Bone asked:
>>>
>>> What is the simplest way to specify the location of text in a
>>> scatter plot
>>> (created using the plot function) in relative terms rather than
>>> specific x-y coordinates? For example, rather than putting text at
>>> (300,49) on a plot, how do I put it 1/10 of the way over from the y
>>> axis and 1/2 of the way up from the x axis? Thanks.
>>>
>> See the help of par:
>> ?par
>>
>> Namely:
>>
>> plot(rnorm(100), rnorm(100))
>> pu <- par()$usr
>> x <- pu[1] * 0.5 + pu[2] * 0.5
>> y <- pu[3] * 0.1 + pu[4] * 0.9
>> text(x, y, "the quick brown fox jumps over a lazy dog")
>>
>> Alberto Monteiro
>
> This is a useful example of placing text, but I interpreted the request 
> as aking for the text to be placed at the left side of the graph (1/10 
> of the x range) and centered vertically (with respect to the y scale). 
> So, then I would calculate x and y as
>
> x <- pu[1] * 0.9 + pu[2] * 0.1
> y <- pu[3] * 0.5 + pu[4] * 0.5

Yes, and there has been another wrong answer earlier.  Note that my 
suggestion of

plot(rnorm(100), rnorm(100))
op <- par(usr=c(0,1,0,1))
text(1/10, 1/2, "some text")
par(op)

is a lot less error-prone (and it also works with log scales).

> Also, text() centers the supplied text at the x,y coordinates. So, if 
> Tom wants the text "begin" at that point, he will need to adjust the x 
> coordinate for the length of the string being printed.

Better, use the adj= argument to text.


-- 
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)
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