Helper utility to get an econodist legend into a ggplot2 plot
add_econodist_legend(gg, legend, below = "subtitle", just = c("left", "center", "right"), legend_height = unit(16, "points"), spacer = unit(10, "points"))
ggplot2 plot object to add
legend grob (any grob, really)
which named gtable element to stick it below?
legend position: "
height of the legend row
height of the spacer that is put below