[patch] add=TRUE in plot.default()

x <- seq(-2, 2, 0.1)
plot(x, x, type="n")
mapply(function(f, i) lines(x, f(x), col=i), functions, 1:3)

-----Original Message-----
From: r-devel-bounces at r-project.org 
[mailto:r-devel-bounces at r-project.org] On Behalf Of Andrew Clausen
Sent: Sunday, March 16, 2008 12:05 AM
To: Duncan Murdoch
Cc: R-devel at r-project.org
Subject: Re: [Rd] [patch] add=TRUE in plot.default()

Hi Duncan,

On Mon, Mar 10, 2008 at 08:51:23AM -0400, Duncan Murdoch wrote:

The add parameter only interacts with other parameters 

superficially 

-- some parameters of "plot" (like log) are related to the 

shape of 

the axes, and should be inherited from what is on the plot already.

I'd say causing some parameters to be completely ignored without a 
warning is more than a superficial interaction.

Fair enough.  I suppose you could inadvertantly put leave 
"add=TRUE" in some code, and wonder why axes aren't getting drawn.

Really, add=TRUE is not
a great design:  it would be better to say "plot methods draw a new 
plot" (so they need args for all the decorations like axes, titles, 
etc.), and have other ways to add things to plots.

I don't like the idea of having a separate way to add to plots.
I like the look of this code:

	f <- function(x) x^2
	g <- function(x) 1/x - 1
	h <- function(x) x
	functions <- c(f, g, h)
	for (i in 1:length(functions))
		plot(functions[[i]], add=i>1, col=i)

Of course, I'd prefer something like

	plot.new()
	for (i in 1:length(functions))
		plot(functions[[i]], add=TRUE, col=i)

but that seems out of reach for compatibility reasons.

How would you write this code in R today?

 Hadley was right on
this, his ggplot2 has a better thought-out design than 

classic graphics. 

  However, we have what we have.

I'm sure ggplot2 is vastly better, although I couldn't find a 
quick intro.
(Typing "ggplot2 examples" into Google didn't help me.)  
Since plot() isn't deprecated yet, it's worth making cheap 
useful improvements.

I agree.  Adding an "add=FALSE" parameter to plot() would generate 
errors for methods that don't implement it, so they would 

all have to 

be changed simultaneously, including in private/unreleased code.

So I'd like to settle for second best: adding add=FALSE 

parameters to 

many plot methods.

I like that suggestion better than adding it here and 

there, one at a 

time.  So, to advance the discussion:  can you take a look 

at the plot 

methods that are in the base and recommended packages, and work out 
how many already have it, how many would be improved if it 

was added, 

and in how many cases it just wouldn't make sense?  (You 

could also do 

this on all the CRAN and Bioconductor packages, but that would be 
about 100 times more work: about 50 times as many packages, 

and much 

less consistency in the programming and documentation standards.  
Maybe on a subset of popular ones, e.g. those that Rcmdr suggests?)

I had a quick look at the base packages.  (I did a crude 
search with find and grep on the R source tarball.)  I 
attached the full list.  There were 40 files
containing plot methods.   Of these, 9 already implemented 
add=TRUE explicitly
and a further 9 inherited it from other plot methods that 
could or already do implement it.  There were 2 methods for 
which it clearly makes no sense.  For example, plot.lm makes 
no sense because it does several separate plots, one after the other.

It looks reasonably straightforward to implement most of the 
remaining methods.
I must confess I don't understand what all of these plots are 
doing from a statistics point of view, but I suppose this 
shouldn't matter.  (Econometrics uses different language...)

Data like that could make a convincing argument that the effort of 
adding this to the base packages is worthwhile.  (To get it 

added to 

non-base packages will require you to convince their 

maintainers that 

it's a good idea.)  Another helpful part of the argument 

will be for 

you to volunteer to do the work of both code and 

documentation updates.

I'm not very excited about doing work outside of base.  I can 
do the base code + documentation though.  Any volunteers?

Cheers,
Andrew