Add geom_hline legend to existing geom bar legend - r

Question

I want to add a legend under the existing legend that represents the dashed line, such that the dashed line could be labeled "avg tx effect" and be placed under study 3.
library(ggplot2)
library(ggthemes)
#dput(df)
df=structure(list(study = structure(c(1L, 2L, 3L, 1L, 2L, 3L, 1L,
2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L), .Label = c("study1", "study2",
"study3"), class = "factor"), d = c(-0.205, 0.1075, 0.3525, -0.37,
0.3, 0.42, -0.28, 0.09, 0.59, 0.11, -0.05, 0.25, 0, 0.25, 0.49
), Outcome = c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L, 4L, 4L, 4L,
5L, 5L, 5L), Outcome2 = structure(c(1L, 1L, 1L, 4L, 4L, 4L, 7L,
7L, 7L, 10L, 10L, 10L, 13L, 13L, 13L), .Label = c("1", "1", "1",
"2", "2", "2", "3", "3", "3", "4", "4", "4", "5", "5", "5"), class = "factor")), .Names = c("study",
"d", "Outcome", "Outcome2"), row.names = c(NA, -15L), class = "data.frame")
ggplot(df, aes(x=Outcome2, y=d, fill=study)) +
geom_bar(position=position_dodge(), aes(x=Outcome2),stat="identity",
colour="black", # Use black outlines,
size=.3) + # Thinner lines
xlab("Outcome") +
ylab("Cohen's D Effect Size") +
scale_fill_grey(name="Study",
labels=c("study1","study2", "study3"))+
theme_bw()+
geom_hline(yintercept=.15,linetype=2)

Answer 1

A general feature of ggplot is that to generate a legend, you need to map your aesthetics (e.g. linetype) to a variable in the data, instead of setting it to constant. In the case of geom_hline, this may be achieved by putting the intercept in a separate data frame. Also note show_guide = TRUE.
Then customize the legend using scale_linetype_manual. The black lines in the fill legend are removed using override.aes.
Here's a stripped down version of your code to show only the most necessary steps:
df2 <- data.frame(yi = 0.15)
ggplot(data = df, aes(x = Outcome2, y = d, fill = study)) +
geom_bar(position = "dodge", stat = "identity") +
geom_hline(data = df2, aes(yintercept = yi, linetype = factor(yi)), show_guide = TRUE) +
scale_linetype_manual(name = "avg tx effect", values = "dashed", labels = "") +
guides(fill = guide_legend(override.aes = list(linetype = "blank")))

Answer 2

As #Gregor suggested, you could use a direct label for this line by adding annotate() as shown below:
ggplot(df, aes(x=Outcome2, y=d, fill=study)) +
geom_bar(position=position_dodge(), aes(x=Outcome2),stat="identity",
colour="black", # Use black outlines,
size=.3) + # Thinner lines
xlab("Outcome") +
ylab("Cohen's D Effect Size") +
scale_fill_grey(name="Study",
labels=c("study1","study2", "study3"))+
theme_bw()+
geom_hline(yintercept=.15,linetype=2) +annotate("text",x=.7,y=.17,size=3,label=c('avg tx ef'))
If space is an issue you can use the wrapper described here to wrap the text. Just run wrapper <- function(x, ...) paste(strwrap(x, ...), collapse = "\n") then add +annotate("text",x=.7,y=.18,size=3,label=wrapper('avg tx effect',10)). Which produces: