Save plot with a given aspect ratio

问题

I\'m working with the really awesome library ggplot2. I figured out how to set the aspect ratio of a plot by using coord_fixed. Now, I\'d like to save the plot to a PDF with a specified width (e.g 10 cm) and let required height get calculated. I did not figure out how to achieve this. Is this even possible?

回答1:

You can use grid functions to calculate the full size of the ggplot grob, but there are (edit: at least) two caveats:

an extra device window will open, to do the unit conversion
the plot panel size will be 0 by default, as it is meant to be calculated on-the-fly according to the device (viewport) it lives in, not the opposite.

That being said, the following function attempts to open a device that fits the ggplot exactly,

library(ggplot2)
library(grid)

sizeit <- function(p, panel.size = 2, default.ar=1){

  gb <- ggplot_build(p)
  # first check if theme sets an aspect ratio
  ar <- gb$plot$coordinates$ratio

  # second possibility: aspect ratio is set by the coordinates, which results in 
  # the use of 'null' units for the gtable layout. let's find out
  g <- ggplot_gtable(gb)
  nullw <- sapply(g$widths, attr, "unit")
  nullh <- sapply(g$heights, attr, "unit")

  # ugly hack to extract the aspect ratio from these weird units
  if(any(nullw == "null"))
    ar <- unlist(g$widths[nullw == "null"]) / unlist(g$heights[nullh == "null"])

  if(is.null(ar)) # if the aspect ratio wasn't specified by the plot
       ar <- default.ar

  # ensure that panel.size is always the larger dimension
  if(ar <= 1 ) panel.size <- panel.size / ar

  g$fullwidth <- convertWidth(sum(g$widths), "in", valueOnly=TRUE) + 
    panel.size
  g$fullheight <- convertHeight(sum(g$heights), "in", valueOnly=TRUE) + 
    panel.size / ar

  class(g) <- c("sizedgrob", class(g))
  g
}


print.sizedgrob <- function(x){
  # note: dev.new doesn't seem to respect those parameters
  # when called from Rstudio; in this case it 
  # may be replaced by x11 or quartz or ...
  dev.new(width=x$fullwidth, height=x$fullheight)
  grid.draw(x)
}


p1 <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() + coord_fixed() +
  theme(plot.background = element_rect(colour = "red"))

p2 <- p1 + aes(x = mpg, y = wt)

# need for an explicit dummy device open, otherwise it's a bit off
# for no apparent reason that I can understand
dev.new() 

sizeit(p1, 0.1)

sizeit(p2, 2)

回答2:

Based on baptiste's answer I stripped his code down to return the aspect ratio as suggested by geotheory. This was much more convenient for me, because I either wanted a fixed width or height and also passed everything through an existing wrapper function that also adds fonts to my pdf.

Oh, and if you used facets you need to take them into account manually. Divide by rows and multiply by columns. Not sure whether there is a better way.....

ggGetAr <- function(p, default.ar=-1){

    gb <- ggplot_build(p)
    # first check if theme sets an aspect ratio
    ar <- gb$plot$coordinates$ratio

    # second possibility: aspect ratio is set by the coordinates, which results in 
    # the use of 'null' units for the gtable layout. let's find out
    g <- ggplot_gtable(gb)
    nullw <- sapply(g$widths, attr, "unit")
    nullh <- sapply(g$heights, attr, "unit")

    # ugly hack to extract the aspect ratio from these weird units
    if(any(nullw == "null"))
        ar <- unlist(g$widths[nullw == "null"]) / unlist(g$heights[nullh == "null"])

    if(is.null(ar)) # if the aspect ratio wasn't specified by the plot
        ar <- default.ar

    ar[1]
}

回答3:

Not sure, but is something like this what you're after?

ggplot(data.frame(x = seq(10), y = seq(10)), aes(x = x, y = y)) +
    geom_point() +
    coord_equal() +
    theme(aspect.ratio = 1)

This looks fine to me:

ggsave("test.pdf", width = 4, height = 4)

Too much white space, but the graphic itself has aspect ratio 1:

ggsave("test2.pdf", width = 4)

Message: Saving 4 x 6.93 in image

回答4:

If you use ggsave you can simply specify the width and height of the graphics device. If you specify the aspect ratio of the plot itself, it is also good to have this aspect ratio (roughly) in your graphics device. The unit of height and width when saving pdf is inches:

ggplot(...) # make a plot here
ggsave("plot.pdf", width = 10)

Now you only have to transform the 10 cm into inches. In addition, height is not forced to a certain aspect ratio if you do not specify it. If you want a 16:9 aspect ratio, you can easily calculate the height based on the width:

ggplot(...) # make plot
width = 10
height = (9/16) * width
ggsave("plot.pdf", width = width, height = height)

You could wrap this in a function if you really want to.

edit: The crux is to synchronize the aspect ratio of the plot (through coord_fixed()) and the aspect ratio of the graphics device. For example

library(ggplot2)
ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() + coord_fixed()
ggsave("plt.png", width = 7, height = 7)

leads to a lot of white space. While the following ggsave call, which has a much better fit in aspect ratio, does not have this amount of white space (sorry for the large picture, could not set the maximum size :)):

ggsave("plt.png", width = 2, height = 7)

回答5:

A more simplistic solution would be to save the plot with default margins and to trim the resulting png with ImageMagick.

require(ggplot2)
require(dplyr)

ggplot(iris, aes(Sepal.Length, Sepal.Width)) + geom_point() + coord_fixed(0.3)
ggsave("untrimmed.png")


system("convert untrimmed.png -trim -bordercolor white -border 20  reframed.png")

For sure the trimming will differ depending on the used output device. E.g. in case of pdf you could use pdfcrop as described here.

来源：https://stackoverflow.com/questions/16422847/save-plot-with-a-given-aspect-ratio

标签

ggplot2

gtable