R Base plotting,如何为多行文字添加粗体层?

R Base plotting, how to add a bold layer to mtext?

mtext("Females", outer=TRUE)

我想将外部标题设为粗体,但无法为其添加粗体层。请帮忙

喜欢this:

x11() 
par(oma = c(0, 0, 2, 0))
plot(1)
mtext("bla", outer = T, font = 2) # font = 2 <- bold