fixedar.r

library(ggplot2)
set.seed(1221)

dat <- data.frame(x=rnorm(100), y=rnorm(100), type=rep(1:2, each=50))
p <- ggplot(dat) + geom_point(aes(x, y)) +
  facet_wrap(~type) + coord_fixed() +
  theme(plot.background = element_rect(fill='red'))

# inspired by
# egg::set_panel_size

g <- ggplotGrob(p)
panels <- grep("panel", g$layout$name)
panel_index_w <- unique(g$layout$l[panels])
panel_index_h <- unique(g$layout$t[panels])
nw <- length(panel_index_w)
nh <- length(panel_index_h)

pw <- g$widths[panel_index_w]  
ph <- g$heights[panel_index_h]

ar <- unlist(pw)[[1]] / unlist(ph)[[1]]

library(grid)
margin <- unit(0.5, 'in')
device_width <- unit(7, 'in') 

# take out the non-null units (panels)
panel_width_in <-  convertWidth(device_width - margin - sum(g$widths), 
                                unitTo = "in", valueOnly = FALSE)
g$widths[panel_index_w] <- 1/nw*panel_width_in

# now set the panel height accordingly
g$heights[panel_index_h] <- 1/ar/nw * panel_width_in

device_height <-  convertHeight(sum(g$heights) + margin, unitTo = "in")

ggsave('test.pdf', g, 
                width = device_width, 
                height = device_height)