R: How do I use coord_cartesian on facet_grid with free-ranging axis

Consider some facet_grid plot

mt <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) + geom_point() 
mt + facet_grid(vs ~ am, scales = "free") 

Imagine I just want to zoom on just the top row in the plots above to only show the y-axes values between 3 and 4. I could do this with coord_cartesian() if they weren't faceted or if I wanted to zoom on all plots, but don't have a good solution in this case. I suppose I could subset the data first, but that is taboo for good reason (e.g. would throw off any statistical layer, etc).

(Note that the question is related to this: R: {ggplot2}: How / Can I independently adjust the x-axis limits on a facet_grid plot? but the answer there will not work for this purpose.)

So, this is an old question, but I wanted to share a way that I found. The simplest seems to be to add artificial points to your data, e.g. at the lower left and top right of your data for each plot. Put this information into a separate data frame, then add it to the plot with geom_point using the parameter alpha=0 to make the points invisible. Set the scaling of the axis in facet_wrap to "free_x" or "free_y", depending on what you need.

Now ggplot scales every facet separately to accommodate the invisible points that you added. A bit hacky, but works nicely.

Old post but i was looking for the same thing and couldn't really find anything - [perhaps this is a way Set limits on y axis for two dependent variables using facet_grid()

The solution is not very elegant/efficient but i think it works - basically create two plots with different coord_cartesian calls and swap over the grobs.

# library(ggplot2)
# library(gtable)

# Plot
mt <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) + geom_point() 

# --------------------------------------------------------------------------------


p1 <- mt + facet_grid(vs ~ am, scales = "free") + coord_cartesian(ylim = c(1,6))
g1 <- ggplotGrob(p1)

p2 <- mt + facet_grid(vs ~ am, scales = "free") + coord_cartesian(ylim = c(3,5))
g2 <- ggplotGrob(p2)

# ----------------------------------------------------------
# Replace the upper panels and upper axis of p1 with that of p2
# Tweak panels of second plot - the upper panels
g1[["grobs"]][[6]] <- g2[["grobs"]][[6]]
g1[["grobs"]][[8]] <- g2[["grobs"]][[8]]

#Tweak axis
g1[["grobs"]][[4]] <- g2[["grobs"]][[4]]

grid.newpage()
grid.draw(g1)