Notches are a very useful feature that can be added to boxplots. They can be added using option “notch=T”, i.e. geom_boxplot(notch=T). The notch indicates confidence interval of the median. The box itself gives interquartile range (ie 25th to 75th percentile), while the ends of lines indicate the range. Any outliers are shown as points beyond the range lines.
Width of the boxes can be made to be variable and indicative of relative sample sizes of different groups: