Creates a plot of Das Gupta standardised rates across the set of populations
dg_plot(dgo, legend.position = "topright")A plot of each of the set of K-a standardised rates across populations
output from dgnpop()
legend position, passed to graphics::legend - choose from "bottomright", "bottom", "bottomleft", "left", "topleft", "top", "topright", "right" and "center", or provide xy.coords