Adding line type to legend in ggplot2

Question

How do I make the line types used by geom_hline or geom_abline show up in the legend of a ggplot plot?

For example:

require (ggplot2)

# some data
dummy <- data.frame (category1 =  rep (1:5, 8), category2 = rep (1:4, each = 10), 
    category3 = rep (factor (1:2), 2), expected = 10 ^ rep (4:7, each = 10),
    value = 10 ^rnorm(40, 5))

# faceted plot
baseplot <-ggplot (dummy ) +
    geom_point (aes (category1, value, color = category3))+
    scale_y_log10 () + 
    facet_wrap (~category2)

# add a dotted line for expected value
p1 <-baseplot + geom_hline ( aes ( yintercept = expected), linetype = 2)

I tried a couple approaches to making the dotted line show up in the legend, but they give me the same thing as p1

p1a < -p1+scale_linetype_discrete (labels = "expected")+
    guides ( linetype= guide_legend ("", labels ="expected"))
p1b <- baseplot + geom_hline (aes (yintercept = expected, linetype = "expected")) +
    scale_linetype_manual (labels= "expected", values = 2)
p1a
p1b

How about multiple lines/line types?

Let's say I also wanted to plot groupwise and overall geometric means

require (reshape)
require (plyr)

# calculate geometric means, keep them in their own data frame
geometric_mean <- function (x)  exp ( mean (log (x)))
dummy $GM_overall <- geometric_mean (dummy $value)
extra <- ddply(dummy, c( "GM_overall", "expected","category2"), summarize,
    GM_group = geometric_mean (value))
extra_long <- melt (GM_group_long, id.vars = "category2")

I expected this approach to show linetype in the legend based on this post, but no such luck

p2=baseplot + geom_hline ( aes ( yintercept = value , linetype = variable), extra)
p2

Here's another case where I would want to do something similar with abline

It would be nice to be able to label the line as 1:1

dummy$value2 <- dummy $value * runif(40, 0.5, 2)
ggplot (dummy)+coord_fixed() +    
    geom_point (aes (value, value2, color = category3))+
    geom_abline (yintercept =0, slope =1) 

I'm using R 3.0.0, ggplot 0.9.3.1

Answer 1

You run through several examples, but this simple case should get you most of the way there:

dummy <- data.frame (category1 =  rep (1:5, 8), category2 = rep (1:4, each = 10), 
    category3 = rep (factor (1:2), 2), expected = 10 ^ rep (4:7, each = 10),
    value = 10 ^rnorm(40, 5))

# faceted plot
baseplot <- ggplot(dummy) +
    geom_point(aes(category1, value, color = category3))+
    scale_y_log10() + 
    facet_wrap(~category2)

# add a dotted line for expected value
baseplot + geom_hline(aes(yintercept = expected,linetype = "expected"),show_guide = TRUE)

The key in most cases, I think, is adding show_guide = TRUE. It is FALSE by default for this geom, which may or may not be intuitive. (I can see the rationale.)

Note how, in this "one line type" case, I "tricked" ggplot into creating a legend by mapping linetype to the character "expected", which causes a new column to be created behind the scenes. Multiple line types should work as expected with the usual methods of creating columns and mapping them to linetype.