theme(
  axis.title = element_text(),         # Change both x and y axis titles
  
  axis.title.x = element_text(),       # Change x axis title only
  axis.title.x.top = element_text(),   # For x axis label on top axis
  
  axis.title.y = element_text(),       # Change y axis title only
  axis.title.y.right = element_text(), # For y axis label on right axis
)