library(ggplot2)
ggplot(diamonds) # if only the dataset is known.
ggplot(diamonds, aes(x=carat)) # if only X-axis is known. The Y-axis can be specified in respective geoms.
ggplot(diamonds, aes(x=carat, y=price)) # if both X and Y axes are fixed for all layers.
ggplot(diamonds, aes(x=carat, color=cut)) # Each category of the 'cut' variable will now have a distinct color, once a geom is added.