r - Set the size of ggsave exactly

Question

R question.

I got so confused by the width, height, dpi and unit.

Why the following two size different?

ggsave(filename = "foo.png",ggplot(mtcars, aes(x=wt, y=mpg)) +
    geom_point(size=2, shape=23),width = 5, height = 4, dpi = 300, units = "in", device='png')

and

ggsave(filename = "foo.png",ggplot(mtcars, aes(x=wt, y=mpg)) +
           geom_point(size=2, shape=23),width = 5, height = 4, dpi = 72, units = "in", device='png')

I set both of the picture's size 5 (inches) * 4 (inches). But why when I change the dpi, the size changes?

How to understand the relationship between height, width, unit and dpi?

Or how to translate these four parameters into unit of pixels, which is easier for me to understand?

Answer 1

To understand why the DPI is important, consider these two plots:

ggsave(filename = "foo300.png", ggplot(mtcars, aes(x=wt, y=mpg)) +
           geom_point(size=2, shape=23) + theme_bw(base_size = 10),
       width = 5, height = 4, dpi = 300, units = "in", device='png')
ggsave(filename = "foo150.png", ggplot(mtcars, aes(x=wt, y=mpg)) +
           geom_point(size=2, shape=23) + theme_bw(base_size = 10),
       width = 10, height = 8, dpi = 150, units = "in", device='png')

The resulting files have the same pixel dimensions, but the font size in each is different. If you place them on a page with the same physical size as their ggsave() calls, the font size will be correct (i.e. 10 as in the ggsave() call). But if you put them on a page at the wrong physical size, the font size won't be 10. To maintain the same physical size and font size while increasing DPI, you have to increase the number of pixels in the image.