Finagling the space and width arguments to barplot to align 2x1 plot window

Question

I'd like to align the bottom barplot in the following so that the groups line up vertically between the two plots:

par(mfrow = c(2, 1))
n = 1:5
barplot(-2:2, width = n, space = .2)

barplot(matrix(-10:9, nrow = 4L, ncol = 5L), beside = TRUE,
        width = rep(n/4, each = 5L), space = c(0, .8))

I've been staring at the definition of the space and width arguments to barplot (from ?barplot) for a while and I really expected the above to work (but clearly it didn't):

width -- optional vector of bar widths. Re-cycled to length the number of bars drawn. Specifying a single value will have no visible effect...

space -- the amount of space (as a fraction of the average bar width) left before each bar. May be given as a single number or one number per bar. If height is a matrix and beside is TRUE, space may be specified by two numbers, where the first is the space between bars in the same group, and the second the space between the groups. If not given explicitly, it defaults to c(0,1) if height is a matrix and beside is TRUE, and to 0.2 otherwise.

As I read it, this means we should be able to match the group widths in the top plot by dividing each group into 4 (hence n/4). For space, since we're dividing each bar's width by 4, the average width will as well; hence we should multiply the fraction by 4 to compensate for this (hence space = c(0, 4*.2)).

However it appears this is being ignored. In fact, it seems all the boxes have the same width! In tinkering around, I've only been able to get the relative within-group widths to vary.

Will it be possible to accomplish what I've got in mind with barplot? If not, can someone say how to do this in e.g. ggplot2?

Answer 1

It is possible to do this with base plot as well, but it helps to pass the matrix as a vector for the second plot. Subsequently, you need to realize the space argument is a fraction of the average bar width. I did it as follows:

par(mfrow = c(2, 1))
widthsbarplot1 <- 1:5
spacesbarplot1 <- c(0, rep(.2, 4))

barplot(-2:2, width = widthsbarplot1, space = spacesbarplot1)

widthsbarplot2 <- rep(widthsbarplot1/4, each = 4)
spacesbetweengroupsbarplot2 <- mean(widthsbarplot2)

allspacesbarplot2 <- c(rep(0,4), rep(c(spacesbetweengroupsbarplot2, rep(0,3)), 4))

matrix2 <- matrix(-10:9, nrow = 4L, ncol = 5L)

barplot(c(matrix2),
    width = widthsbarplot2,
    space = allspacesbarplot2,
    col = c("red", "yellow", "green", "blue"))

Answer 2

You can actually pass widths in ggplot as vectors as well. You'll need the dev version of ggplot2 to get the correct dodging though:

library(dplyr)
library(ggplot2)

df1 <- data.frame(n = 1:5, y = -2:2)
df1$x <- cumsum(df1$n)
df2 <- data.frame(n = rep(1:5, each = 4), y2 = -10:9)
df2$id <- 1:4                                                    # just for the colors

df3 <- full_join(df1, df2)

p1 <- ggplot(df1, aes(x, y)) + geom_col(width = df1$n, col = 1)
p2 <- ggplot(df3, aes(x, y2, group = y2, fill = factor(id))) + 
  geom_col(width = df3$n, position = 'dodge2', col = 1) +
  scale_fill_grey(guide = 'none')

cowplot::plot_grid(p1, p2, ncol = 1, align = 'v')

Answer 3

Another way, using only base R and still using barplot (not going "down" to rect) is to do it in several barplot calls, with add=TRUE, playing with space to put the groups of bars at the right place.

As already highlighted, the problem is that space is proportional to the mean of width. So you need to correct for that.

Here is my way:

# draw first barplot, getting back the value
bp <- barplot(-2:2, width = n, space = .2)

# get the xlim
x_rg <- par("usr")[1:2]

# plot the "frame"
plot(0, 0, type="n", axes=FALSE, xlab="", ylab="", xlim=x_rg, xaxs="i", ylim=range(as.vector(pr_bp2)))

# plot the groups of bars, one at a time, specifying space, with a correction according to width, so that each group start where it should
sapply(1:5, function(i) barplot(pr_bp2[, i, drop=FALSE], beside = TRUE, width = n[i]/4, space = c((bp[i, 1]-n[i]/2)/(n[i]/4), rep(0, 3)), add=TRUE))

Answer 4

You can do this in ggplot2 by setting the x-axis locations of the bars explicitly and using geom_rect for plotting. Here's an example that's probably more complicated than it needs to be, but hopefully it will demonstrate the basic idea:

library(tidyverse)

sp = 0.4

d1 = data.frame(value=-2:2) %>% 
  mutate(key=paste0("V", 1:n()),
         width=1:n(),
         spacer = cumsum(rep(sp, n())) - sp,
         xpos = cumsum(width) - 0.5*width + spacer)

d2 = matrix(-10:9, nrow = 4L, ncol = 5L) %>% 
  as.tibble %>% 
  gather(key, value) %>%
  mutate(width = as.numeric(gsub("V","",key))) %>% 
  group_by(key) %>% 
  mutate(width = width/n()) %>% 
  ungroup %>% 
  mutate(spacer = rep(cumsum(rep(sp, length(unique(key)))) - sp, each=4),
         xpos = cumsum(width) - 0.5*width + spacer)

d = bind_rows(list(d1=d1, d2=d2), .id='source') %>% 
  group_by(source, key) %>% 
  mutate(group = LETTERS[1:n()])

ggplot(d, aes(fill=group, colour=group)) +
  geom_rect(aes(xmin=xpos-0.5*width, xmax=xpos+0.5*width, ymin=0, ymax=value)) +
  facet_grid(source ~ ., scales="free_y") +
  theme_bw() +
  guides(fill=FALSE, colour=FALSE) +
  scale_x_continuous(breaks = d1$xpos, labels=d1$key)