transparency using plot+points with sp classes

Hi guys,
Thansk for the help with the transparency.

I've made some modifications to my code , added rgb in colour to play
with the transparency and included also a north arrow. I was trying to
make the code look better, however i am facing another problem, once I
use symbols with a bg colour. Somehow once I plot the map I loose the
bg colour info and he gives me only the filled triangle border colour:

arrow<- list("SpatialPolygonsRescale", layout.north.arrow(type=1),
offset = c(5,10), scale = 2,col="black")

list1=list("sp.points",observations2000,pch=21,col = rgb(red = 0,
green = 0, blue = 1, alpha = 0.1))

list2=list("sp.points,"observations1999,pch=21,col="blue")

list3=list("sp.points",reference.points,pch=24,col="red",bg="orange")

spplot(polygon_area,"NAME",col="black",col.regions=FALSE, colorkey=FALSE,

        sp.layout=list(arrow,list1,list2,list3),

        scales=list(draw=TRUE),xlab="Longitude",ylab="Latitude", main="Map" )



On Wed, Dec 28, 2011 at 1:13 PM, Ana<rrasterr at gmail.com>  wrote:

How can I make one point graphics with transparency

These are all sp classes:

plot(polygons_area,axes=TRUE,asp=1.5,main="Title",xlab="Latitude",
ylab="Longitude")
points(observations2000,type = "p",pch=21,col="green")
points(observation1999,type = "p",pch=21,col="blue")
points(reference.points,type = "p",pch=21,col="red")

how can I add transparency to one of the point graphics, for example
points(observation1999,type = "p",pch=21,col="blue")  ?