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There are various ways to set the formatting of labels in plots. First and foremost, you may want to change the label text:

ggplot(data=iris, 
       mapping=aes(x=Sepal.Length, 
                  y=Petal.Length, color=Species))+
  ggtitle("Iris")+
  geom_point()+
  gg_("set the x-axis label to 'This is my x-axis'")+
  gg_("The y-axis label should be set to 'Welcome to the jungle', please")

Rotation

ggplot(data=iris, 
       mapping=aes(x=Sepal.Length, 
                  y=Petal.Length, color=Species))+
  ggtitle("Iris")+
  geom_point()+
  gg_("Rotate the x-axis labels by 32°")+
  gg_("Rotate the y-axis labels by 172 degrees")

Colors

ggplot(data=iris, 
       mapping=aes(x=Sepal.Length, 
                  y=Petal.Length, color=Species))+
  ggtitle("Iris")+
  geom_point()+
  gg_("Paint the x-axis label in coral.")+
  gg_("The label of the y-axis should appear in purple.")