The function summarise_layout() returns a table that provides information about
the plot panel(s) in the built plot. The table has the following columns:
panel
A factor indicating the individual plot panels.
row
Row number in the grid of panels.
col
Column number in the grid of panels.
vars
A list of lists. For each panel, the respective list
provides the variables and their values that specify the panel.
xmin, xmax
The minimum and maximum values of the variable mapped to
the x aesthetic, in transformed coordinates.
ymin, ymax
The minimum and maximum values of the variable mapped to
the y aesthetic, in transformed coordinates.
xscale
The scale object applied to the x aesthetic.
yscale
The scale object applied to the y aesthetic.
Importantly, the values for xmin, xmax, ymin, ymax, xscale, and yscale
are determined by the variables that are mapped to x and y in the aes() call.
So even if a coord changes how x and y are shown in the final plot (as is the case
for coord_flip() or coord_polar()), these changes have no effect on the results
returned by summarise_plot().