egg package

This document provides a brief overview of the main utiilty functions included in the egg package.

Exposing ggplot2 layouts

p1 <- qplot(mpg, wt, data = mtcars, colour = cyl)
p2 <- qplot(mpg, data = mtcars) + ggtitle("title")
p3 <- qplot(mpg, data = mtcars, geom = "dotplot")
p4 <- p1 + facet_wrap( ~ carb, nrow = 1) + theme(legend.position = "none") +
  ggtitle("facetted plot")
pl <- lapply(list(p1, p2, p3, p4), expose_layout, FALSE, FALSE)
grid.arrange(
  grobs = pl,
  widths = c(1.2, 1, 1),
  layout_matrix = rbind(c(1, 2, 3),
                        c(4, 4, 4))
)

Setting panel size

p1 <- qplot(mpg, wt, data = mtcars, colour = cyl)
p2 <- p1 + facet_wrap( ~ carb, nrow = 1)
grid.arrange(grobs = lapply(
  list(p1, p2),
  set_panel_size,
  width = unit(2, "cm"),
  height = unit(1, "in")
))

Aligning complex ggplots

This is a convenience function similar to grid.arrange, specialised for ggplot2 objects, which attempts to align the plot panels,

p1 <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) +
  geom_point() +
  guides(colour = "none") +
  theme_bw() + theme(strip.background = element_blank())

p2 <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) +
  geom_point() + facet_wrap(~ cyl, ncol = 2, scales = "free") +
  guides(colour = "none") +
  labs(title = "Title", subtitle = "subtitle") +
  theme_bw() + theme(strip.background = element_rect(fill = "transparent"))

ggarrange(p1, p2, widths = 1:2, debug = TRUE)

Widths and heights should correspond to the layout.

p <- ggplot()
ggarrange(p, p, p, widths = c(3, 1), heights = c(5, 1))

For convenience, labels can be added to refer to the subfigures. All parameters of textGrob can be used for the formatting of the labels, including the positioning (x, hjust, etc.).

ggarrange(p1, p2, p3, ncol=2,
          labels = c("A", "b)", "iii."), 
          label.args = list(gp=gpar(font=4), 
                            x=unit(1,"line"), hjust=0))

Custom geom

The function geom_custom extends the ggplot2 function annotation_custom to cases where multiple grobs are to be placed, e.g. on different panels, or at different positions in a plot. This geom is a bit special in that it does not truly respect a grammar of graphics – arbitrary grobs can be plotted, with no explicit mapping to variables. Its typical use would be to place annotations (images, tables, …). The data used to create the annotation is passed as a list-column.

codes <- data.frame(country = c("nz","ca","ar","fr","gb","es"))
codes$y <- runif(nrow(codes))

gl <- lapply(codes$country, 
                    function(.x) png::readPNG(system.file("flags", 
                                             paste0(.x,".png"),
                                             package="egg")))

codes$raster <- I(gl)


ggplot(codes, aes(x = country, y = y)) + 
  geom_point() +
  geom_custom(data = codes, aes(data=raster), 
              grob_fun = rasterGrob, 
              fun_params = list(height=unit(1,"cm")))