Decreasing space between legend columns in ggplot2 - r

Question

Here is some example code, which provides a legend with 2 columns. I want to decrease the space between the two colums of the legend (see below).
library(ggplot2)
labels <- c(expression(""^13*CH[4]),
expression(""^13*CH[4]~"+"~SO[4]^{2-''}),
expression(""^13*CH[4]~"+"~MoO[4]^{2-''}))
ggplot(aes(mpg, wt, colour = factor(cyl), shape=factor(cyl)),
data = mtcars) +
geom_point() +
scale_colour_manual(values=c("red", "green", "blue"), label=labels)+
scale_shape_manual(values = c(4,5,6), label=labels)+
theme(legend.position = "bottom",
legend.text.align = 0,
legend.text = element_text(size=8),
legend.key.size = unit(0.8, 'lines')) +
guides(col = guide_legend("", ncol=2), shape=guide_legend("", col=2))
Here is my real life problem:
Additional space is needed on the right side of the plot, because the three factor levels there contain much more characters. However, i am really constrained in the plot size. Hence, I would like to decrease the space between the two rows of the legend.
I also would like to keep the most bottom factor level of the left hand side as is, without adding an extra line.

Answer 1

Based on your example, I simplified it a bit:
Create the problematic plot:
library(ggplot2)
labels <- c("short1", "loooooooooooooooooong", "short2")
plt <- ggplot(aes(mpg, wt, colour = factor(cyl), shape=factor(cyl)),
data = mtcars) +
geom_point() +
scale_colour_manual(values=c("red", "green", "blue"), label=labels)+
scale_shape_manual(values = c(4,5,6), label=labels)+
theme(legend.position = "bottom",
legend.text.align = 0,
legend.text = element_text(size=8),
legend.key.size = unit(0.8, 'lines')) +
guides(col = guide_legend("", ncol=2), shape=guide_legend("", col=2))
plot(plt)
Extract the legend and tweak it
I used this answer to extract the legend from the plot:
#Extract Legend
g_legend<-function(a.gplot){
tmp <- ggplot_gtable(ggplot_build(a.gplot))
leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
legend <- tmp$grobs[[leg]]
return(legend)}
legend <- g_legend(plt)
And print it:
grid.newpage()
grid.draw(legend)
Then I explored the grobs inside the legend and I found the widths field:
legend$grobs[[1]]$widths
[1] 0.2cm 0cm 0.1524cm 0.4064cm 0.0762cm 3.22791666666667cm 0.0762cm 0.4064cm 0.0762cm
[10] 0.79375cm 0.2cm
>
Apparently those 3.227 cm are too much so I just changed them:
legend$grobs[[1]]$widths[6] <- unit(1.5, "cm")
And plot it:
grid.newpage()
grid.draw(legend)
Apply the fix to the global plot:
The final steps are to replicate that on the ggplot:
Apply that same manual correction to the global plot:
# this is how the legend was extracted:
plt_gtable <- ggplot_gtable(ggplot_build(plt))
leg <- which(sapply(plt_gtable$grobs, function(x) x$name) == "guide-box")
# Replace the legend with our modified legend:
plt_gtable$grobs[[leg]] <- legend
And replot:
grid.newpage()
grid.draw(plt_gtable)