Frequently, we’ll want to customize our axes to represent our data more clearly. For discrete variables, such as categories on the x axis of a bar plot, we may want to specify a particular order these values appear in. Or, we might want to rename the axes labels so they better describe each value.

We can customize discrete variables on the x axis with the scale_x_discrete() layer. There is also a scale_y_discrete() layer that works the same way for variables on the y axis.

Let’s take a look at the hours of sleep by diet plot we created in the last exercise. We can pass a vector to the limits argument in scale_x_discrete() to specify that we only want to show bars for c("omni", "carni", "herbi"), in that order rather than the default alphabetical ordering.

msleep_start <- 
  ggplot(msleep_error_df, 
         aes(x = diet, 
             y = mean.hours)) + 
  geom_bar(stat = "identity") + 
  geom_errorbar(aes(ymin = se.min, 
                    ymax = se.max), 
                width = 0.2) + 
  labs(title = "Mean Hours Asleep by Diet")

msleep_discrete <- 
  msleep_start +
  scale_x_discrete(
    limits = c("omni", "carni", "herbi"))

This produces the following plot. "insecti" is now omitted on the x axis. "omni" appears on the left, whereas it would appear on the right in the default alphabetical ordering.

We might also want the labels to be more descriptive – for example, "carnivore" is a more familiar word compared to "carni". We can pass a vector of value-to-label mappings to the labels argument in scale_x_discrete() to specify what we want to show for each existing value.

msleep_discrete <- 
  msleep_start + 
  scale_x_discrete(
    limits = c("omni", "carni", "herbi"), 
    labels = c("carni" = "Carnivore", 
               "herbi" = "Herbivore", 
               "omni" = "Omnivore"))

This produces the following plot. Now the labels are much more clear! The ordering we specify in limits is still preserved.