You want to change the appearance of labels in a legend.

Use theme(legend.text=element_text()) (Figure 10-16):

# The base plot
p <- ggplot(PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot()

# Change the legend label appearance
p + theme(legend.text=element_text(face="italic", family="Times", colour="red", 
                                   size=14))

Figure 10-16. Customized legend label appearance

It’s also possible to specify the legend label appearance via guides(), although this method is a bit unwieldy. This has the same effect as the previous code:

# Changes the legend title text for the fill legend
p + guides(fill=guide_legend(label.theme=
           element_text(face="italic", family="Times", colour="red", size=14)))