> str(city_frequency)
'data.frame':   144 obs. of  4 variables:
 $ city: Factor w/ 144 levels "A Coruna","Agen",..: 1 2 3 4 5 6 7 8 9 10 ...
 $ freq: int  7 2 4 1 4 2 1 1 1 5 ...
 $ lon : num  -8.412 0.616 -8.446 -4.262 2.149 ...
 $ lat : num  43.4 44.2 40.6 42.8 43.9 ...

xlim <- c(-13.08, 8.68)
ylim <- c(34.87, 49.50)
map("world", lwd=0.05, xlim=xlim, ylim=ylim)
symbols(city_frequency$lon, city_frequency$lat, circles=sqrt(city_frequency$freq), bg="#93ceef", fg="#ffffff", lwd=0.5, inches=0.15, add=TRUE)

m <- get_map("France", zoom = 5, maptype = "toner")
ggmap(m) + geom_point(data = city_frequency, aes(lon = lon, lat = lat, size = sqrt(freq)), alpha = 0.7) + labs(x = "longitude", y = "latitude", title = "City Frequency")