Error: no stat called StatHline

I have a data frame as follows:

variable=c("D","D","C","C","C","A","B","B","B","B")
value=c(80,100,70,68,65,45,33,31,36,32)
Count=as.integer(c(5,10,4,5,2,7,3,5,6,2))
mean=c(93.3,93.3,68.2,68.2,68.2,45,33.4,33.4,33.4,33.4)
sumVarVal=data.frame(variable=variable,value=value,Count=Count,mean=mean)

I can make a nice plot (where the size of the square corresponds to the count of observations with that particular x-value and y-value), as shown below:

library(ggplot2)
ggplot(sumVarVal, aes(variable, value)) + 
  geom_point(aes(size = Count), pch=15) + 
  guides(fill=guide_legend(title="New")) + 
  theme(legend.background = element_rect(fill="gray90", 
        size=.5, 
        colour = "black"), 
        legend.text=element_text(size=rel(1.3)), 
        legend.title=element_text(size=rel(1.3), face="plain"), 
        legend.position="bottom", 
        axis.text = element_text(size=rel(1.3)), 
        axis.title = element_text(size = rel(1.3))) + 
  labs(x="Learning Outcome", y = "Percentage Grade")

However, I used to have an additional piece of code (at the end of the syntax) that allowed me to superimpose a horizontal bar on each of the four topics, indicating the mean percentage grade. Those values are stored in df$mean. Here is the code I was using:

ggplot(sumVarVal, aes(variable, value)) +
  geom_point(aes(size = Count), pch=15) + 
  guides(fill=guide_legend(title="New")) + 
  theme(legend.background = element_rect(fill="gray90", size=.5, colour = "black"), 
        legend.text=element_text(size=rel(1.3)), 
        legend.title=element_text(size=rel(1.3), face="plain"), 
        legend.position="bottom", 
        axis.text = element_text(size=rel(1.3)),
        axis.title = element_text(size = rel(1.3))) + 
  labs(x="Learning Outcome", y = "Percentage Grade") + 
  geom_errorbar(stat = "hline", width=0.6, colour = "blue", size = 1, aes(ymax=..y..,ymin=..y.., yintercept = mean))

With version 1.0.1, this gives:

With version 2.0.0, it now leads to an error:

Error: no stat called StatHline.

I know this may be connected to recent upgrades in ggplot2. I have seen other recent comments about it (geom_errorbar - "No stat called StatHline"). However, due to my code surrounding the use of stat="hline", when I tried some of these suggestions, I was not able to get my code to work either. Perhaps there is something I do not understand about my original code that is preventing me from being able to update this issue?

EDIT: I have taken into account some of the suggestions, and am currently using this code:

ggplot(sumVarVal, aes(variable, value)) +
  geom_point(aes(size = Count), pch=15) + 
  guides(fill=guide_legend(title="New")) + 
  theme(legend.background = element_rect(fill="gray90", size=.5, colour = "black"), 
        legend.text=element_text(size=rel(1.3)), 
        legend.title=element_text(size=rel(1.3), face="plain"), 
        legend.position="bottom", 
        axis.text = element_text(size=rel(1.3)),
        axis.title = element_text(size = rel(1.3))) + 
  labs(x="Learning Outcome", y = "Percentage Grade") + 
  geom_errorbar(stat = "summary", fun.y = "mean", width=0.6, colour = "blue", size = 1, aes(ymax=..y..,ymin=..y.., yintercept = mean))

This gives me an output that looks like this:

It seems that some of the mean blue lines are not lining up to their values, as given originally in the mean vector. For instance, for variable "D", it should have a mean value of 93.3, but the blue horizontal line seems to be displayed at a value of 90.0. The effect is even more dramatic in my real code (not this MWE). Any ideas what might be causing this discrepancy?

stat_hline got removed in ggplot2 2.0.0, but never fear; it wasn't really necessary anyway. If you remove the stat argument entirely, it will default to identity, which is fine. (summary can work, too, if you prefer.) You need to change the aes mapping, though, changing yintercept to y to account for the new stat.

All together,

ggplot(sumVarVal, aes(variable, value)) +
  geom_point(aes(size = Count), pch=15) + 
  guides(fill=guide_legend(title="New")) + 
  theme(legend.background = element_rect(fill="gray90", size=.5, colour = "black"), 
        legend.text=element_text(size=rel(1.3)), 
        legend.title=element_text(size=rel(1.3), face="plain"), 
        legend.position="bottom", 
        axis.text = element_text(size=rel(1.3)),
        axis.title = element_text(size = rel(1.3))) + 
  labs(x="Learning Outcome", y = "Percentage Grade") + 
  geom_errorbar(width=0.6, colour = "blue", size = 1, aes(ymax=..y.., ymin=..y.., y = mean))

produces