[Rd] suggest enhancement to segments and arrows to facilitate horizontal and vertical segments

[Prof Brian Ripley]

This isn't a one-way street, as it means that some erroneous calls 
will not be detected, and code written to take advantage of this will 
only work under R >= 2.10.0.

I think we should probably insist that one of x1 and y1 is supplied: 
if both are omitted arrows() would give a warning, but segments() 
would not.

On Fri, 2 Oct 2009, Tim Hesterberg wrote:

> I suggest a simple enhancement to segments() and arrows() to
> facilitate drawing horizontal and vertical segments --
> set default values for the second x and y arguments equal to the first set.
> This is handy, especially when the expressions for coordinates are long.
>
> Compare:
>
> Segments:
> < function (x0, y0, x1 = x0, y1 = y0, col = par("fg"), lty = par("lty"),
> ---
>> function (x0, y0, x1, y1, col = par("fg"), lty = par("lty"),
>
> Arrows:
> < function (x0, y0, x1 = x0, y1 = y0, length = 0.25, angle = 30, code = 2,
> ---
>> function (x0, y0, x1, y1, length = 0.25, angle = 30, code = 2,
>
> Tim Hesterberg

-- 
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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