it is density curves, and he shall use ggplot geom_density() function where you will have R dataframe

ggplot(yourdataFrame, aes(yourTrait, fill = variable_to_distinguish_betweentrait, colour = variable_to_distinguish_betweentrait)) +geom_density(alpha = 0.5)

good example can be found here https://www.r-bloggers.com/density-plot-with-ggplot/