R ggplot2: How to plot a mean line for 2 groups across subgroup boxplots?

Question

I have 13 sites that are classified into 2 groups, and I have plotted a boxplot of values for each site using ggplot2. They are identified by group based on the fill color. How can I add a mean line that shows the mean of the two groups, across the site boxplots for each group? (ie one mean line across the boxes for group 1, one line across group 2)

I have tried using "stat_summary()" but can only get it to plot lines for each mean for the individual sites, not the mean across the whole groups. (Individual mean lines are shown in image, NOT what I want)

ggplot(data, aes(y = value, x=site, fill=group)) +
  geom_boxplot() + labs(y="value")+
  theme_classic() +
  theme(legend.position="bottom", 
        axis.title.x=element_blank(),
        axis.ticks.x=element_blank(), 
        text=element_text(size=15)) +
  scale_fill_manual(values=c("#E69F00", "#56B4E9"), name="group", 
                    labels=c("1", "2")) +
  stat_summary(fun.y = mean, geom = "errorbar", 
               aes(ymax = ..y.., ymin = ..y.., group =factor(mgmt)),
               width = 0.75, linetype = "dashed")

I expect 2 mean lines, one for each group, that extend across the site boxes for each group. Instead I get a mean line for each site box.

Answer 1

例如geom_hline将使您在整个图表上成为一条水平线， geom_segment可以使它更短（ 前者 ， 后者 ）。