R/ggplot2 - Overlapping labels on facet_grid

Question

Folks,

I am plotting histograms using geom_histogram and I would like to label each histogram with the mean value (I am using mean for the sake of this example). The issue is that I am drawing multiple histograms in one facet and I get labels overlapping. This is an example:

library(ggplot2)
df <- data.frame (type=rep(1:2, each=1000), subtype=rep(c("a","b"), each=500), value=rnorm(4000, 0,1))
plt <- ggplot(df, aes(x=value, fill=subtype)) + geom_histogram(position="identity", alpha=0.4)
plt <- plt +  facet_grid(. ~ type)
plt + geom_text(aes(label = paste("mean=", mean(value)), colour=subtype, x=-Inf, y=Inf), data = df, size = 4, hjust=-0.1, vjust=2)

Result is:

The problem is that the labels for Subtypes a and b are overlapping. I would like to solve this.

I have tried the position, both dodge and stack, for example:

plt + geom_text(aes(label = paste("mean=", mean(value)), colour=subtype, x=-Inf, y=Inf), position="stack", data = df, size = 4, hjust=-0.1, vjust=2)

This did not help. In fact, it issued warning about the width.

Would you pls help ? Thx, Riad.

Answer 1

I think you could precalculate mean values before plotting in new data frame.

library(plyr)
df.text<-ddply(df,.(type,subtype),summarise,mean.value=mean(value))

df.text
  type subtype   mean.value
1    1       a -0.003138127
2    1       b  0.023252169
3    2       a  0.030831337
4    2       b -0.059001888

Then use this new data frame in geom_text(). To ensure that values do not overlap you can provide two values in vjust= (as there are two values in each facet).

ggplot(df, aes(x=value, fill=subtype)) + 
  geom_histogram(position="identity", alpha=0.4)+
  facet_grid(. ~ type)+
  geom_text(data=df.text,aes(label=paste("mean=",mean.value),
                 colour=subtype,x=-Inf,y=Inf), size = 4, hjust=-0.1, vjust=c(2,4))