This function is a direct interface to the circle packing algorithm used by
`layout_tbl_graph_circlepack`

. It takes a vector of sizes and
returns the x and y position of each circle as a two-column matrix.

`pack_circles(areas)`

- areas
A vector of circle areas

A matrix with two columns and the same number of rows as the length
of the "areas" vector. The matrix has the following attributes added:
"enclosing_radius" giving the radius of the smallest enclosing circle, and
"front_chain" giving the terminating members of the front chain (see
Wang *et al*. 2006).

Wang, W., Wang, H. H., Dai, G., & Wang, H. (2006). *Visualization of
large hierarchical data by circle packing*. Chi, 517-520.

```
library(ggforce)
sizes <- sample(10, 100, TRUE)
position <- pack_circles(sizes)
data <- data.frame(x = position[,1], y = position[,2], r = sqrt(sizes/pi))
ggplot() +
geom_circle(aes(x0 = x, y0 = y, r = r), data = data, fill = 'steelblue') +
geom_circle(aes(x0 = 0, y0 = 0, r = attr(position, 'enclosing_radius'))) +
geom_polygon(aes(x = x, y = y),
data = data[attr(position, 'front_chain'), ],
fill = NA,
colour = 'black')
```