[Rd] documenation for arrows() is backwards (PR#7839)

[Peter Dalgaard]

hoffman.mm+R-project.org at ebi.ac.uk writes:

> Full_Name: Michael Hoffman
> Version: 2.1.0
> OS: Linux (Fedora Core 3, kernel 2.6.11-1.14_FC3)
> Submission from: (NULL) (193.62.199.8)
> 
> 
> help(arrows) says:
> 
> """
>      arrows(x0, y0, x1, y1, length = 0.25, angle = 30, code = 2,
>             col = par("fg"), lty = NULL, lwd = par("lwd"), xpd = NULL)
> 
> ...
> 
>      If 'code=2' an arrowhead is drawn at '(x0[i],y0[i])' and if
>      'code=1' an arrowhead is drawn at '(x1[i],y1[i])'.  If 'code=3' a
>      head is drawn at both ends of the arrow.  Unless 'length = 0',
>      when no head is drawn.
> """
> 
> If you do:
> 
> plot(c(-1, 1), c(-1, 1), col=0)
> arrows(x0=0, y0=0, x1=0, y1=1, code=2)
> 
> it is pretty clear that the arrowhead is drawn at x1, y1 with code=2. If you
> switch to code=1, the arrowhead is drawn at x0, y0. Either the documentation or
> the function is incorrect.

One way to decide is to compare with Splus. Now that doesn't have a
'code' argument and the arguments are named x1,y1,x2,y2 (I wonder why
R wanted to be different here?), but they do put the arrowheads at the
*to* end, which does seem to be the sensible thing to do. 

Arguably, using 'code=2' as the default is a bit weird, but changing
it could be quite painful. I.e., we should fix the docs.

BTW, the docs also might say that code=0 makes arrows behave like
segments(). It is also a bit strange that the code argument isn't
vectorized, which might have been useful. 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907