Using ggplot2 for this kind of work is much easier, you can add points, polygons, 2densities, etc... to ggmap layers.
library(RgoogleMaps)
library(sp)
library(ggplot2)
library(ggmap)
P
is the SpatialPointsDataFrame object:
DB <- data.frame(FID=P$FID, city=P$city)
DB <- cbind(DB, P@coords)
DB <- data.frame(FID=c(0,1), city=c("New York", "Newark"), POINT_X=c(-73.996786,-74.172237), POINT_Y=c(40.720813,40.732196 ))
gc <- geocode("new york, usa")
center <- as.numeric(gc)
G <- ggmap(get_googlemap(center = center, color = 'bw', scale = 4), extent = "device")
G1 <- G + geom_point(aes(x=POINT_X, y=POINT_Y ),data=DB, color="red", size=5)
plot(G1)
This is the output: