Create a map of the United States that combines geographic spatial dimensions and data size

I am creating a map of the United States that highlights some specific states defined by a categorical variable. What I would like to do is inject the size of the data into these states. Something similar to a geographic histogram, such as in the image below ;

So far, I've just gotten to a 2D level using ggplot. I've read about mapping in RGL 3D environment, but I'm not sure how to implement it.

ggplot(data = map.data, aes(x = long, y = lat, group = group)) +
    geom_polygon(aes(fill = expansion, alpha = fpl.cat)) +
    scale_fill_manual(values = c('#004487', '#91AF5F', '#F09628'), guide=FALSE) +
    scale_alpha_discrete(range = c(.4, 1), guide=FALSE) +
    geom_path(colour = 'black', linestyle = 3) +
    coord_map()