ggplot - add box on top of graph

I have the following data and graph:

 fixed <- as.data.frame(rep(0.125,8))
fixed[,2] <- c("1","2","3","4","5","6","7","8")
colnames(fixed) <- c("Percentage", "Test")

Is it possible to add the mean as a separate box on top of the graph? I do not want to share the same y-axis, as this can be confusing with the label.

Here is an example of what I am looking for. It would be great if I can paint the background of the box darker as well, is that possible:

Here is the code for my plot:

p <- ggplot(fixed,aes(x=Test ,y=Percentage,fill=Test))+
  geom_boxplot()+ 
  stat_summary(fun.data = function(y) 
    data.frame(y=0.6, label = paste(round(mean(y),2))), geom="text",size=3) +
  geom_hline(yintercept=0.55, linetype="dashed", color = "black")+
  theme(legend.position="none")+
  scale_y_continuous(limits = c(0, 0.6),breaks=seq(0,0.6,0.1),
                     labels= c("0%","10%","20%","30%","40%","50%","Mean"))+
  theme_bw()+
  labs(title = "Title")+
  xlab("Test")+
  ylab("Percetage")+
  theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank(),
        panel.background = element_rect(fill = "grey90", colour = NA),
        plot.background = element_rect(fill = "grey90", colour = NA),
        legend.position="none",plot.title = element_text(size= 12, hjust=0.5),
        plot.margin = unit(c(1,1,1,1), "cm"))+
  scale_fill_manual(values=c("#00441b","#006d2c","#238b45","#41ab5d","#74c476",
                             "#a1d99b","#c7e9c0","#e5f5e0"))
p

Solution

fixed <- as.data.frame(rep(0.125, 8))
fixed[, 2] <- c("1", "2", "3", "4", "5", "6", "7", "8")
colnames(fixed) <- c("Percentage", "Test")

Let's add some, er, structure to the ggplot2 build. And, we're going to move some data and aes() directly to stat_s and geom_s so we can do the box, er, geom_rect() annotation. Comments are in/near salient lines:

ggplot() +
  geom_boxplot(
    data = fixed, aes(x = Test, y = Percentage, fill = Test)
  ) +
  geom_rect( # HERE IS UR BOX
    data = data.frame(),
    aes(xmin = -Inf, xmax = Inf, ymin = 0.565, ymax = Inf),
    size = 2.5, fill = "#00000000", color = "black"
  ) +
  stat_summary(
    data = fixed, aes(Test, Percentage),
    fun.data = function(y) {
      data.frame(
        y = 0.6,
        label = paste(round(mean(y), 2))
      )
    },
    geom = "text", size = 3
  ) +
  scale_y_continuous(
    limits = c(0, 0.6), breaks = seq(0, 0.6, 0.1),
    labels = c("0%", "10%", "20%", "30%", "40%", "50%", "Mean")
  ) +
  scale_fill_manual(
    values = c(
      "#00441b", "#006d2c", "#238b45", "#41ab5d", 
      "#74c476", "#a1d99b", "#c7e9c0", "#e5f5e0"
    )
  ) +
  coord_cartesian(clip = "off") + # WE NEED TO TURN CLIPPING OFF
  labs(
    x = "Test", y = "Percentage", title = "Title"
  ) +
  theme_bw() +
  theme(
    legend.position = "none", # YOU HAD THIS IN A SEPARATE theme(). IT PAYS TO HAVE A CODE STYLE
    panel.grid.major = element_blank(),
    panel.grid.minor = element_blank(),
    panel.background = element_rect(fill = "grey90", colour = NA),
    plot.background = element_rect(fill = "grey90", colour = NA),
    plot.title = element_text(size = 12, hjust = 0.5),
    plot.margin = unit(c(1, 1, 1, 1), "cm"),
    axis.text.y = element_text( # MAKE THE "Mean" BIG & BOLD 
      size = c(rep(10, 6), 20), face = c(rep("plain", 6), "bold")
    )
  )