uk cities with transparency mapped to population size

UKCities.R

library(ggplot2)
library(magrittr)
library(dplyr)
library(maps)

data("world.cities")

world.cities %>% 
  filter(country.etc == "UK") %>% 
ggplot(aes(x = long, y = lat, alpha = pop)) + 
  geom_point() + 
  coord_map()