The default ggplot axis titles are the variable names:

library(ggplot2)
ggplot(mtcars, aes(cyl, wt))  
  geom_point()

^{Created on 2021-09-10 by the}

CodePudding user response：

I am not sure what your reason is to put it as a subtitle. Perhaps it is more worthwhile to simply move the position of the x-axis label?

ggplot(mtcars, aes(cyl, wt))  
  geom_point()   
  scale_x_discrete(position = "top")