Different size facets at x-axis

Question

Length of x-axis is important for my plot because it allows one to compare between facets, therefore I want facets to have different x-axis sizes. Here is my example data:

group1 <- seq(1, 10, 2)
group2 <-  seq(1, 20, 3)
x = c(group1, group2)
mydf <- data.frame (X =x , Y = rnorm (length (x),5,1), 
                    groups = c(rep(1, length (group1)), rep(2, length(group2))))

And my code:

p1 = ggplot(data=mydf,aes(x=X,y=Y,color=factor(groups)) )+
  geom_point(size=2)+
  scale_x_continuous(labels=comma)+
  theme_bw()
p1+facet_grid(groups ~ .,scales = "fixed",space="free_x")

And the resulting figure:

Panel-1 has x-axis values less then 10 whereas panel-2 has x-axis value extending to 20. Still both panels and have same size on x-axis. Is there any way to make x-axis panel size different for different panels, so that they correspond to their (x-axis) values?

I found an example from some different package that shows what I am trying to do, here is the figure:

Answer 1

Maybe something like this can get you started. There's still some formatting to do, though.

library(grid)
library(gridExtra)
library(dplyr)
library(ggplot2)

p1 <- ggplot(data=mydf[mydf$groups==1,],aes(x=X,y=Y))+
  geom_point(size=2)+
  theme_bw()
p2 <- ggplot(data=mydf[mydf$groups==2,],aes(x=X,y=Y))+
  geom_point(size=2)+
  theme_bw()

summ <- mydf %>% group_by(groups) %>% summarize(len=diff(range(X)))
summ$p <- summ$len/max(summ$len)
summ$q <- 1-summ$p

ng <- nullGrob()
grid.arrange(arrangeGrob(p1,ng,widths=summ[1,3:4]),
             arrangeGrob(p2,ng,widths=summ[2,3:4]))

I'm sure there's a way to make this more general, and the axes don't line up perfectly yet, but it's a beginning.

Answer 2

Here is a solution following OP's clarifying comment ("I guess axis will be same but the boxes will be of variable size. Is it possible by plotting them separately and aligning in grid?").

library(plyr); library(ggplot2)

buffer <- 0.5 # Extra space around the box

#Calculate box parameters
mydf.box <- ddply(mydf, .(groups), summarise,
      max.X = max(X) + buffer,
      min.X = 0,
      max.Y = max(Y) + buffer,
      min.Y = 0,
      X = mean(X), Y = mean(Y)) #Dummy values for X and Y needed for geom_rect


p2 <- ggplot(data=mydf,aes(x=X, y=Y) )+ 
  geom_rect(data = mydf.box, aes( xmax = max.X, xmin = min.X, 
                                  ymax = max.Y, ymin = min.Y),
            fill = "white", colour = "black", fill = NA) +
  geom_point(size=2) +  facet_grid(groups ~ .,scales = "free_y") +
  theme_classic() +
  #Extra formatting to make your plot like the example
  theme(panel.background = element_rect(fill = "grey85"),
        strip.text.y = element_text(angle = 0),
        strip.background = element_rect(colour = NA, fill = "grey65"))