[R] transparency using plot+points with sp classes

Ana rrasterr at gmail.com
Wed Dec 28 13:13:14 CET 2011

How can I make one point graphics with transparency

These are all sp classes:

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")  ?

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