[R] Creating png of layered legend

[Amelia McNamara]

I am trying to create a plot that has multiple plot characters for
each point (e.g. a point within a triangle, a triangle within a
square, etc). The workaround I have found to do this is by plotting
twice, as in this example:

x <- c(1.1, 2.3, 4.6)
y <- c(2.0, 1.6, 3.2)
plot(x, y)
points(x,y, pch=20, col="red", cex=0.5)

This works, but perhaps there is a better way to do it in one step? My
main problem comes when I want to make a legend. The workaround I have
here is to again plot twice, as in

legend(x="bottomright", legend=c("Category 1"), text.col="white",
title="Fairly long title", title.col="white", col=c("red"),pch=20,
pt.cex=0.5, bty="o", bg="white")
legend(x="bottomright", legend=c("Category 1"), title="Fairly long
title", pch=1, bty="o")

which produces the correct image in my Quartz window. I have to
include the title and legend text in both versions, plotting the first
one in white to avoid the text getting overplotted while still
creating a legend border of the appropriate size.

However, when I try to save this as a png, only the most recently
plotted legend is captured. Saving as a pdf works, but the deliverable
for this project is a png file and I would like to be able to produce
them directly from R. Is there a way for me to capture both legends in
a png? Or is there a better way for me to plot multiple symbols for a
single point?

Amelia McNamara