[ggplot] evaluate the ratio between width and height of a plot with "coord_equal"


I create plots for automated reports with ggplot and officer packages.
Some plots have a coord_equal() constraint.
In order to add my plot to my docx report without unwanted margins due to a bad choice of height and width, I would like to evaluate first the ratio between the height and the width (of the entire plot including legend and title).

Plot example:


g <- ggplot(mtcars, aes(mpg, qsec, colour = disp, shape = factor(carb))) +
  geom_point() +
  coord_equal() +
  theme(legend.position = "top",
        plot.margin = margin(1, 1, 1, 1, unit = "lines")) +

The ratio of this example height / width is visually equal to 0.55.
It's more evident with gtable that show the entire plot layout in blue:

g_g <- ggplotGrob(g)

I have not found in the list of ggplotGrob(g) nor in gtable_show_layout(g_g) any way to find the ratio between height and width. gtable_show_layout know however to print it correctly !
When I try to sum the height of rows and width of cols and convert it, the results are quite confusing.

h <- grid::convertUnit(gtable::gtable_height(g_g), 'inches', valueOnly = TRUE)
w <- grid::convertUnit(gtable::gtable_width(g_g), 'inches', valueOnly = TRUE)
h/w    # = 2.27

# with absolute.size
h <- grid::convertUnit(grid::absolute.size(gtable::gtable_height(g_g)),
                       'inches', valueOnly = TRUE)
w <- grid::convertUnit(grid::absolute.size(gtable::gtable_width(g_g)),
                       'inches', valueOnly = TRUE)
h/w    # = 2.79

Anybody can tell me where I'm wrong ?

Thanks, Gilles

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