[R] Overlaying maps

[Michael.Laviolette at dhhs.state.nh.us]

I'm having difficulty overlaying maps when writing to a file graphics
device. My command sequence has the structure

plot(map1)
par(new = T)
plot(map2)

On the screen device, it works fine. When I attempt something like

png(file = "map.png")
plot(map1)
par(new = T)
plot(map2)
dev.off()

only the last map appears, the previous ones having been cleared. Can
someone clarify?

Thanks,
Michael Laviolette PhD MPH
New Hampshire Department of Health and Human Services