We say in ?points that 'pch' (among others) can be set to NA
for omitting a point.
While this works in cases where there's at least one point left
to draw, it fails in a simple case like this :
plot(1, pch = NA)
Error in plot.xy(xy.coords(x, y), type = type, pch = pch, col = col, bg = bg, :
invalid plotting symbol
Both in R-patched or R-devel.
A simple workaround {hinting at how to fix the C code} is
plot(1, pch = as.integer(NA))
So this is easily fixable (and I'll commit a patch soon).
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A bit more problematic *) but not a bug in a very strict sense
is the following :
> plot(1, pch = as.character(NA))
and
> plot(1:4, pch=c("o",".", NA, "x"))
which are identical to
> plot(1, pch = "N")
and
> plot(1:4, pch=c("o",".", "N", "x"))
but should either produce a warning (additional to using "N") or
rather also treat the NA as "missing", i.e., not drawing a point.
Martin Maechler
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*) because this isn't spotted when used in a context of
plotting many things