R help - Jul 2013 - legend in ggmap

Hello,

how can I visualize the legend in the following ggmap plot ? In the legend
I just want to show the size ranges of the data:

ggmap(map) + geom_point(aes(x=Longitude,y=Latitude), size = dataRd[,2]/12,
col=2, data=dataRd, alpha=0.7, lwd=2)


Thanks

Edoardo

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