Justification of multiple legends in ggmap/ggplot2

Question

I am trying to make a map with two legends denoting shape and colour ("Type" and "Org" in the example below), and have the legends inset. I can place the legends, but I would like them to be left justified so that their left edges line up. I can't make them anything other than centred with respect to each other:

require(ggplot2)
require(ggmap)
require(grid)
require(mapproj)

data <- data.frame(Org=rep(c("ABCDEFG","HIJKLMNOP","QRSTUVWX"),4)
                   , Type=rep(c("Y","Z"),6), Lat=runif(12,48,54.5)
                   , Long=runif(12,-133.5,-122.5))

osmMap <- get_map(location=c(-134,47.5,-122,55), source = 'osm')

points <- geom_jitter(data=data, aes(Long, Lat, shape=Type
                                     , colour=Org))

legend <- theme(legend.justification=c(0,0), legend.position=c(0,0)
                , legend.margin=unit(0,"lines"), legend.box="vertical"
                , legend.key.size=unit(1,"lines"), legend.text.align=0
                , legend.title.align=0)

ggmap(osmMap) + points + legend

Hartmann

This isn't a direct answer but you can cheat a bit by using format to make them equivalently sized. Try data$Type <- format(data$Type, width=17) after your data declaration and rerun your code. – Gillmore

Answer 1

This option is now available in ggplot2 0.9.3.1, use

ggmap(osmMap) + points + legend + theme(legend.box.just = "left")

Old, manual solution:

Here is a solution:

require(gtable)
require(ggplot2)
require(ggmap)
require(grid)
require(mapproj)

# Original data
data <- data.frame(Org=rep(c("ABCDEFG","HIJKLMNOP","QRSTUVWX"),4),
                   Type=rep(c("Y","Z"),6), Lat=runif(12,48,54.5),
                   Long=runif(12,-133.5,-122.5))
osmMap <- get_map(location=c(-134,47.5,-122,55), source = 'google')
points <- geom_jitter(data=data, aes(Long, Lat, shape=Type, colour=Org))
legend <- theme(legend.justification=c(0,0), legend.position=c(0,0),
                legend.margin=unit(0,"lines"), legend.box="vertical",
                legend.key.size=unit(1,"lines"), legend.text.align=0,
                legend.title.align=0)

# Data transformation
p <- ggmap(osmMap) + points + legend
data <- ggplot_build(p)
gtable <- ggplot_gtable(data)

# Determining index of legends table
lbox <- which(sapply(gtable$grobs, paste) == "gtable[guide-box]")
# Each legend has several parts, wdth contains total widths for each legend
wdth <- with(gtable$grobs[[lbox]], c(sum(as.vector(grobs[[1]]$widths)), 
                                     sum(as.vector(grobs[[2]]$widths))))
# Determining narrower legend
id <- which.min(wdth)
# Adding a new empty column of abs(diff(wdth)) mm width on the right of 
# the smaller legend box
gtable$grobs[[lbox]]$grobs[[id]] <- gtable_add_cols(
                                      gtable$grobs[[lbox]]$grobs[[id]], 
                                      unit(abs(diff(wdth)), "mm"))
# Plotting
grid.draw(gtable)

This does not depend on Type or Org. However, this would not be enough having more than two legends. Also, in case you do some changes so that list of grobs (graphical objects) is altered, you might need to change grobs[[8]] to grobs[[i]] where i is the position of your legends, see gtable$grobs and look for TableGrob (5 x 3) "guide-box": 2 grobs.

Edit: 1. Automatically detecting which grob is legends table, i.e. no need to change anything after modifying other parts of plot. 2. Changed calculation of width differences, now code should work when having any two legends, i.e. in more complex cases as well, for example:

Hopson

I'm unable to reproduce this example. Where do I get osmMap from? I assume this is some data set. – Ancheta

@FaheemMitha, yes, that is data from the question. I have added it to my answer and also changed source = 'osm' to source = 'google' since the former doesn't work at least for now. – Hopson