Shade rectangular areas of a ggplot plot across facets

shading.R

## It took me three days to find this solution!  

## <https://stackoverflow.com/questions/44688623/adding-custom-images-to-ggplot-facets/44897816#44897816>
annotation_custom2 <- function (grob, xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf, data) 
    {
        layer(data = data, stat = StatIdentity, position = PositionIdentity, 
              geom = ggplot2:::GeomCustomAnn,
              inherit.aes = FALSE, params = list(grob = grob, 
                                                 xmin = xmin, xmax = xmax, 
                                                 ymin = ymin, ymax = ymax))
    }

rects = tibble(
    Species = c('setosa', 'versicolor', 'virginica'), 
    low = c(-1, 1.5, 2), 
    high = c(1, 2, 3), 
    fill = c(.1, .2, .3)
) %>% 
    mutate(grob = pmap(list(low, high, fill, Species), 
                       ~ annotation_custom2(grid::rectGrob(gp = grid::gpar(fill = 'black', alpha = ..3)), 
                                           xmin = -Inf, xmax = Inf, 
                                           ymin = ..1, ymax = ..2, 
                                           data = tibble(Species = ..4))))

p = ggplot(iris, aes(Species, Petal.Width)) +
    geom_point() +
    facet_wrap(~ Species) + 
    coord_flip()
p + rects$grob