Re: [R] How to use the function "plot" as Matlab?

From: Ted Harding <Ted.Harding_at_nessie.mcc.ac.uk>
Date: Wed 13 Jul 2005 - 18:12:42 EST


On 13-Jul-05 klebyn wrote:
> Hello,
>
> How to use the function plot to produce graphs as Matlab?
> example in Matlab:
>
> a = [1,2,5,3,6,8,1,7];
> b = [1,7,2,9,2,3,4,5];
> plot(a,'b')
> hold
> plot(b,'r')
>
>
> How to make the same in R-package ?
>
> I am trying something thus:
>
> a <- c(1,2,5,3,6,8,1,7)
> c(1,7,2,9,2,3,4,5) -> b
>
> a;b
>
> plot(a,t="l",col="blue")
> plot(b,t="l",col="red")

Although this is an over-worked query -- for which an answer, given that t="l" has been specified, is to use

  plot(a,t="l",col="blue",ylim=c(0,10))
  lines(b,t="l",col="red")

there is a more interesting issue associated with it (given that Klebyn has come to it from a Matlab perspective).

It's a long time since I used real Matlab, but I'll illustrate with octave which, in this respect, should be identical to Matlab.

Octave:

octave:1> x = 0.1*(0:20);
octave:2> plot(x,sin(x))

produces a graph of sin(x) with the y-axis scaled from 0 to 1.0 Next:

octave:3> hold on
octave:4> plot(x,1.5*cos(x))

superimposes a graph of 1.5*cos(x) with the y-axis automatically re-scaled from -1 to 1.5.

This would not have happened in R with

  x = 0.1*(0:20);
  plot(x,sin(x))
  lines(x,1.5*cos(x))

where the 0 to 1.0 scaling of the first plot would be kept for the second, in which therefore part of the additional graph of 1.5*cos(x) would be "outside the box".

No doubt like many others, I've been caught on the wrong foot by this more than a few times. The solution, of course (as illustrated in the reply to Klebyn above) is to anticipate what scaling you will need for all the graphs you intend to put on the same plot, and set up the scalings at the time of the first one using the options "xlim" and "ylim", e.g.:

  x = 0.1*(0:20);
  plot(x,sin(x),ylim=c(-1,1.5))
  lines(x,1.5*cos(x))

This is not always feasible, and indeed should not be expected to be feasible since part of the reason for using software like R in the first place is to compute what you do not know!

Indeed, R will not allow you to use "xlim" or "ylim" once the first plot has been drawn.

So in such cases I end up making a note (either on paper or, when I do really serious planning, in auxiliary variables) of the min's and max's for each graph, and then re-run the plotting commands with appropriate "xlim" and "ylim" scaling set up in the first plot so as to include all the subsequent graphs in entirety. (Even this strategy can be defeated if the succesive graphs represent simulations of long-tailed distributions. Unless of course I'm sufficiently alert to set the RNG seed first as well ... )

I'm not sufficiently acquainted with the internals of "plot" and friends to anticipate the answer to this question; but, anyway, the question is:

  Is it feasible to include, as a parameter to "plot", "lines"   and "points",

    rescale=FALSE

  where this default value would maintain the existing behaviour   of these functions, while setting

    rescale=TRUE

  would allow each succeeding plot, adding graphs using "points"   or "lines", to be rescaled (as in Matlab/Octave) so as to   include the entirety of each successive graph?

Best wishes to all,
Ted.



E-Mail: (Ted Harding) <Ted.Harding@nessie.mcc.ac.uk> Fax-to-email: +44 (0)870 094 0861
Date: 13-Jul-05                                       Time: 09:12:34
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