[R] filled plot symbols

Ross Ihaka ihaka at stat.auckland.ac.nz
Wed Oct 18 21:09:36 CEST 2000

On Wed, Oct 18, 2000 at 09:02:29AM +0100, Bill Simpson wrote:
> On Tue, 17 Oct 2000 ben at zoo.ufl.edu wrote:
> >   As for the wider question of why there are two different kinds of
> > points, etc., etc. -- I think this is basically historical, and makes more
> > sense if you think about the R graphics model, which is just objects drawn
> > onto a canvas. pch=1 doesn't fill in the symbol with any color at all,
> > while pch=16 fills it in.  If you want quick access to filled and unfilled
> > symbols, having pch 1 and 16 (and all their neighbors) is easier than
> > fussing with background colors.  
> I don't see the reason for having the unfillable versions of the plot
> symbols (1-6). If you do 
> plot(x,y,type="o",pch=21)
> you get transparent circles and if you do
> plot(x,y,type="o",pch=21,bg="white")
> you get opaque circles. So no need for pch=1 (which can only do
> transparent circles).
> In any case, I don't see why anyone would want the transparent symbols. It
> makes for a messy plot whenever there is overlap. Occlusion is better than
> overlap.

Depends what you are doing.  The use of a transparent circle is deliberate
and follows a recommendation of Bill Cleveland -- it makes the overlap
of symbols as about obvious as possible.  We often judge the density
of points by the amount of ink in an area.  If that is the goal, then
erasing any ink is a bad idea.

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