Combine ggplots but fix the size/ratio of the plot

Question

I have two plots that I combine. arrangeGrob() squeezes them so that the size of the new image is the same as one alone. How can I arrange them while preserving the ratio/size?

require(ggplot2)
require(gridExtra)

dat <- read.csv("http://www.ats.ucla.edu/stat/data/fish.csv")

frqncy <- as.data.table(table(dat$child))#
frqncy$V1 <- as.numeric(frqncy$V1)

plot1 <- ggplot(frqncy, aes(x=V1, y= N)) +
    geom_histogram(stat="identity", binwidth = 2.5)
plot2 <- ggplot(frqncy, aes(x=V1, y= N)) +
    geom_density(stat="identity")

plot <- arrangeGrob(plot1, plot2)

Plot looks like

I have not found any parameter in ggplot() or arrangeGrob() that fixes the ratio of the input.

Edit: Additional complications arise from the definition of axis labels in arrangeGrob(), i.e.

plot <- arrangeGrob(plot1, plot2, left="LHS label")

Then the new file will not automaticall shrink to the minimum height/width combination of plot1 and plot2.

Answer 1

You can control this when you output to a device. For example, a PDF file:

pdf("plot.pdf", width=5,height=8)
plot
dev.off()

Another option is to set a fixed ratio between the x and y coordinates in the plot itself using coord_fixed(ratio=n), where n is the y/x ratio. This will set the relative physical length of the x and y axes based on the nominal value range for each axis. If you use coord_fixed() the graph will always maintain the desired aspect ratio no matter what device size you use for your output.

For example, in your case both graphs have x-range 0 to 3 and y-range 0 to 132. If you set coord_fixed(ratio=1), your graphs will be tall and super skinny because the x-axis length will be 3/132 times the y-axis length (or to put it another way, 1 x-unit will take up the same physical length and 1 y-unit, but there are only 3 x-units and 132 y-units). Play around with the value of ratio to see how it works. A ratio of somewhere around 0.02 is probably about right for your graphs.

As an example, try the following code. Here I've set the ratio to 0.1, so now 1 x-unit takes up 10 times the physical length of each y-unit (that is, 0 to 3 on the x-axis has the same physical length as 0 to 30 on the y-axis).

plot1 <-ggplot(frqncy, aes(x=V1, y= N)) +
  geom_histogram(stat="identity", binwidth = 2.5) + 
  coord_fixed(ratio=0.1)
plot2 <- ggplot(frqncy, aes(x=V1, y= N)) +
  geom_density(stat="identity") +
  coord_fixed(ratio=0.1)

plot <- arrangeGrob(plot1, plot2)

pdf("plot.pdf", 5,8)
plot
dev.off()

Answer 2

there are several other options, depending on what you want*

library(ggplot2)
p = qplot(1, 1)
grid.arrange(p, p, respect=TRUE) # both viewports are square
grid.arrange(p, p, respect=TRUE, heights=c(1,2)) # relative heights

p1 = p + theme(aspect.ratio=3)
grid.arrange(p,p1, respect=TRUE) # one is square, the other thinner

*: the aspect ratio is often not a well-defined property of plots (unless set manually), because the default is to extend the plot to the available space defined by the plot window/device/viewport.