[R] Getting plot axes where they should be!

(Ted Harding) Ted.Harding at manchester.ac.uk
Mon Jul 21 15:01:01 CEST 2008

On 21-Jul-08 12:43:47, Duncan Murdoch wrote:
> On 7/21/2008 8:13 AM, (Ted Harding) wrote:
>> Hi Folks,
>> I've been digging for the solution to this for several
>> hours now. If there is a solution, it must be one of the
>> worst "needle-in-a-haystack" examples in R documentation!
>> Essentially, I want to make an x-y plot in which the
>> X-axis really is the X-axis (i.e. its vertical position
>> is at y=0), and the Y-axis really is the Y-axis (i.e.
>> its horizontal position is at x=0). Discussion, with
>> toy examples, below.
>> I have sort-of solved this (as stated) for one special
>> case, after a depth-4 search through
>>   ?plot --> ?plot.default --> ?par --> ?axis
>> which finally led me to the parameter "pos" to axis():
>>   ?axis
>>   pos: the coordinate at which the axis line is to be drawn:
>>   if not 'NA' this overrides the values of both 'line' and 'mgp[3]'.
>> Hence, instead of
>>   plot(c(0.5,2.5),c(0.5,2.5),xlim=c(0,3),ylim=c(0,3),
>>        frame.plot=FALSE)
>> (where the axes do not meet at the origin (0,0)), I can do
>>   plot(c(0.5,2.5),c(0.5,2.5),xlim=c(0,3),ylim=c(0,3),
>>        frame.plot=FALSE,pos=0)
>> which is *exactly* what I want in this case.
>> But now I want to do the same, where instead of plotting the
>> two points (0.5,0.5), (2.5,2.5) I want to plot (0.5,2.5), (2.5,4.5).
>> Provided I keep the xlim and ylim to both have lower value 0,
>> a similar solution again works fine:
>>   plot(c(0.5,2.5),c(2.5,4.5),xlim=c(0,3),ylim=c(0,5),
>>        frame.plot=FALSE,pos=0)
>> But, in this case, what I *really* want is to limit the Y range
>> to the "relevant" bit: ylim=c(2,5) -- I don't want to have a lot
>> of empty space below the points. So I want a Y-axis running from
>> y=2 to y=5, and X-axis as before from x=0 to x=3, and I want these
>> two axes to meet at (x=0,y=2). But how?
>> By analogy to the above, I need to set a "pos=0" for the X-axis,
>> and a "pos=2" for the y-axis. And I have not been able to discover
>> how to do this.
> It may or may not be possible in a single call to plot(), but it is 
> certainly straightforward if you use separate calls to plot() and axis:
>  > plot(c(0.5,2.5),c(2.5,4.5),xlim=c(0,3),ylim=c(2,5), axes=F)
>  > axis(1, pos=2)
>  > axis(2, pos=0)

Thanks, Duncan! That nicely rounds off Marc's response, and makes
sense (and also, in fact, exposes my "blind spot" when I was reading
the documentation in the first place).

> Generally speaking I find it is usually easier not to try to convince 
> plot() to do strange things:  I tell it to do nothing, and do the 
> strange things myself.

It depends what you mean by "strange", Duncan. To be frank, I find
R's default "offset" axes to be strange, and what I've been trying
to achieve to be "normal". But that's a matter of taste, I suppose
-- unless you need, for instance, to be able to lay a ruler over
the plot and see where it meets the "axis"!

> Duncan Murdoch


E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 21-Jul-08                                       Time: 14:00:58
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