Click on the city to see population

library(leaflet)
md_cities <- data.frame(name = c("Barcelona", "Girona", "Lleida", "Tarragona"),
       pop = c(5562188,748636,428418,795571),
       pop2 = c("5,562,188","748,636","428,418","795,571"),
       lng = c(2.176349270,2.824118990,0.620619340,1.258421900),
      lat = c(41.384246640,41.981860750,41.615273550,41.119102870))

md_cities %>%
        leaflet() %>%
        addTiles() %>%
        addCircles(lng = md_cities$lng, lat = md_cities$lat,
                weight = 3, radius = sqrt(md_cities$pop) * 15,
                   popup = md_cities$pop2, )