Set file background from plot theme

[karawoo]

Closes #4057. @clauswilke I took your proposed approach and calculated the theme element to get the background color. I imagine people will still typically want to edit their theme to remove the plot border.

I'm opening this as a draft because I think it needs a test but I'm not sure how best to write one. Is there a way to have vdiffr compare two files rather than comparing a plot object against a file? I imagine there is and that vdiffr is doing this under the hood normally, but I tinkered with it a bit and couldn't figure it out.

library("ggplot2")

p <- ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  coord_fixed() + # using coord_fixed() just to emphasize what's the plot and what's the image background
  theme(plot.background = element_rect(fill = "forestgreen"))

ggsave("~/p_new.png", p)
#> Saving 7 x 5 in image
knitr::include_graphics("~/p_new.png")

ggsave("~/p_override.png", p, bg = "cyan")
#> Saving 7 x 5 in image
knitr::include_graphics("~/p_override.png")

[clauswilke]

You could write the png to a temp file, read it back in, plot as a rasterGrob() with grid, and then vdiffr can compare that.

[karawoo]

I couldn't get the rasterGrob() method to work, so instead of writing a visual test I'm saving to svg and finding the background rect with xpath. This involves adding xml2 to Suggests which maybe we don't want to do for just one test. If you all prefer I can write a regex instead, that may be ok for this purpose.

[clauswilke]

What was the problem with the rasterGrob()approach? Maybe it can be fixed? Could you post the code you tried?

[clauswilke]

Thinking some more about this, I think this is a reasonable approach.

[karawoo]

Thanks @clauswilke, and you feel ok about adding xml2 to Suggests?

[clauswilke]

I realized that writing an image and reading it back in to plot would also require an additional dependency (e.g. png), so it's a wash. And in any case just writing to and reading from svg (rather than writing a png, reading it in, plotting to svg, and then comparing that svg to a reference) is much more efficient. And also, vdiffr imports xml2, so we have this dependency already, implicitly.