R ggplot - set legend/axis sizes with ggsave

Question

so I have df like this

df <- read.table(text="
   amount      nr      date
   50          1       2017-01-01
   150         1       2017-01-03
   1500        2       2017-01-04
   1450        2       2017-01-04
   1250        2       2017-01-04
   950         1       2017-02-05
   120         3       2017-02-06
   300         3       2017-04-06
", header=TRUE)

I created an plot from this df

ggplot(test, aes(x = date, y = amount, fill = nr, group = 1)) +
  geom_bar(stat = "identity")

Everything works well until I save this plot with code below

ggsave(filename="D:/Documents/units_plot.png", width = 4, height = 2)

I can't figure out how should I properly set image dimensions (it should be 1000x500px) and how to set text size for legends, axes text sizes. Usually I do it by theme() option i ggplot but for some reason legend is so huge in exported file.

Answer 1

ggplot bases its size calculations around the physical size of the image once it's printed. To get a certain pixel size, you need to pick a good value for DPI (number of pixels per inch), and use that to calculate the size:

ggplot(test, aes(x = date, y = amount, fill = nr, group = 1)) +
    geom_bar(stat = "identity") +
    theme_grey(base_size = 12)

dpi = 96
ggsave(filename="units_plot.png", width = 1000 / dpi, height = 500 / dpi,
       dpi = dpi)

If you pick different DPI values, you'll have to test which font sizes work with that DPI, without making the labels too big or small - I don't think there's any way around that.