Problems with plot and Quartz device / What is alpha?

BTW: you may want to use something like pch=19, cex=0.1 (and maybe add some 
alpha to get a quick density estimation).

Cheers,
S

So I guess it might be related to the
"special treatment" of "." described on help(points) ??
---
Value pch="." (equivalently pch = 46) is handled specially. It is a 
rectangle of
side 0.01 inch (scaled by cex).
---

I find this behaviour unnerving: the (resized) plots made it look as if 
there
was a lot of structure in the data, but on closer inspection it turned out 
to be
entirely a consequence of the quartz device and plot function!

I can't imagine that this behaviour is intended - even if it were intended 
to
suppress points (like some axis labels) - it seems strange that enlarging 
the
window makes points disappear. (I also tried setting dpi=72 in quartz(), 
but
this did not fix the problem).

Thanks in advance.

Thomas

------------------------------------------------------
sessionInfo()
R version 2.9.0 (2009-04-17)
i386-apple-darwin8.11.1

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base