plot_spacer in nested compositions interferes with alignment and figure size

[ilia-kats]

Consider this example (slightly modified from the docs):

from plotnine import ggplot, aes, geom_point, geom_boxplot, theme, element_rect
from plotnine.composition import plot_spacer, Beside, Stack
from plotnine.data import mtcars

p1 = ggplot(mtcars, aes("wt", "mpg")) + geom_point()

p = Stack([Beside([p1, p1, p1]), Beside([p1, p1, plot_spacer()])])
p + theme(figure_size=(10, 10))

This results in

where the figure_size is completely ignored (not really visible here in the issue, but evident in a notebook when trying different figure sizes).

On the other hand, getting rid of plot_spacer restores correct alignment and figure sizing:

p = Stack([Beside([p1, p1, p1]), Beside([p1, p1])])
p + theme(figure_size=(10, 10))

p = Stack([Beside([p1, p1, p1]), Beside([p1, p1, p1])])
p + theme(figure_size=(10, 10))

This is on plotnine 0.15.0.

[has2k1]

There are 2 issues.

1. When you compose any two compositions, only edges of panels that are on the outside are guaranteed to align. This will be solved in an upcoming release by the + operator, which will make it possible to wrap plots into a grid.

   p = Stack([Beside([p1, p1, p1]), Beside([p1, p1, p1])])
   p + theme(figure_size=(10, 10))

   Despite what that looks like it is not a 2 x 3 grid. A grid will need something link
   p1 + p1 + p1 + p1 + p1 + p1 + plot_layout(ncol=3, nrow=2). And plot_spacer will
   surprise less when used in a grid.

2. Changing the size of the figure is impossible when you have plot_spacer() in the last spot. (This is a bug).