library(ggplot2)

#example stored in baseplot
ID <- 1:400
values1 <- rnorm(200, mean=50, sd=10) 
values2 <- c(rnorm(100, mean=20, sd=10), rnorm(100, mean=40, sd=10))
df <- data.frame(ID, values1, values2)
baseplot <- ggplot(df, aes(values1, values2)) +
  geom_point(size=2) 
baseplot

#changing axis titles with xlab() and ylab()
baseplot +
  xlab("variable1 (unit)") +
  ylab("variable2 (unit)")


#changing axis titles with scale_x_continuous() and scale_y_continuous()
baseplot +
  scale_x_continuous("variable3 (unit)") +
  scale_y_continuous("variable4 (unit)")

#changing the size and color of both axis titles with theme(axis.title = element.text())
baseplot +
  xlab("variable1 (unit)") +
  ylab("variable2 (unit)") +
  theme(axis.title = element_text(color = "blue", size = 12))

#changing the size and color of only one of the axis titles with theme(axis.title.x = element.text()) and theme(axis.title.y = element.text())
baseplot +
  xlab("variable1 (unit)") +
  ylab("variable2 (unit)") +
  theme(axis.title.x = element_text(color = "blue", face ="bold", size = 12)) +
  theme(axis.title.y = element_text(color = "red", size = 14))