Add category mean value to faceted scatter plots in ggplot - r

Question

I am using facet wrap to plot Weight Gain versus Caloric Intake for four different diets. Diet is a four-level factor, Weight Gain and Caloric Intake are numeric. I am adding a regression line to each plot facet. What I want to do is add a horizontal line for the group mean weight gain for each diet in the plot (4 different mean values). The problem is when I use the geom_hline function it puts the global mean on all of the plots, which is not what I want.
I tried using stat_summary(fun.y=mean,geom="line"), but it gives me line segments joining each of the points in every plot.
Below is the code I am using that is giving me the single global mean on all plots. Also the data set I am using. I've included the labeller code for completeness but I really just need help with drawing the group mean lines.
Thanks in advance for any help.
# Calculate slopes and means to use for facet labels
#
wgSlope<-rep(NA,nlevels(vitaminData$Diet))
dietMeans<-rep(NA,nlevels(vitaminData$Diet))
for (i in 1:nlevels(vitaminData$Diet)){
dietMeans[i]<-mean(filter(vitaminData,Diet==i)$WeightGain)
#
# Get regression lines and coefficients for each facet
#
lm<-lm(WeightGain~CaloricIntake,data=filter(vitaminData,Diet==i))
wgSlope[i]<-lm$coefficients[2]
}
#
# Build facet labels
#
dietLabel<-c(`1`=
paste("Diet 1, Slope=",round(wgSlope[1],2),", Mean=",round(dietMeans[1],1)),
`2`=paste("Diet 2, Slope=",round(wgSlope[2],2),", Mean=",round(dietMeans[2],1)),
`3`=paste("Diet 3, Slope =",round(wgSlope[3],2),", Mean=",round(dietMeans[3],1)),
`4`=paste("Diet 4, Slope =",round(wgSlope[4],2),", Mean=",round(dietMeans[4],1)))
#
# Draw the plots
#
ggplot(data=vitaminData,
aes(y=WeightGain,x=CaloricIntake,color=Diet))+
theme_bw()+
geom_point(aes(color=Diet,fill=Diet,shape=Diet))+
geom_smooth(method="lm",se=FALSE,linetype=2,alpha=0.5)+
labs(x="Caloric Intake",y="Weight Gain")+
scale_color_manual(values=c("red","blue","orange","darkgreen"))+
geom_hline(yintercept=mean(vitaminData$WeightGain))+
facet_wrap(~Diet,labeller=labeller(Diet=dietLabel))+
theme(legend.position="none")
Diet WeightGain CaloricIntake
<fct> <dbl> <dbl>
1 1 48 35
2 1 67 44
3 1 78 44
4 1 69 51
5 1 53 47
6 2 65 40
7 2 49 45
8 2 37 37
9 2 73 53
10 2 63 42
11 3 79 51
12 3 52 41
13 3 63 47
14 3 65 47
15 3 67 48
16 4 59 53
17 4 50 52
18 4 59 52
19 4 42 45
20 4 34 38

Answer 1

Here's an approach using dplyr. (Add library(dplyr) or library(tidyverse) if not already loaded.)
geom_hline(data = vitaminData %>%
group_by(Diet) %>%
summarize(mean = mean(WeightGain)),
aes(yintercept = mean)) +