The following code used to work but doesn't anymore. Does anybody have an idea what's going on? It must be some change in the underlying gtable code.
require(cowplot) # for plot_grid() require(grid) # for unit_max() # make two plots plot.iris <- ggplot(iris, aes(Sepal.Length, Sepal.Width)) + geom_point() + facet_grid(. ~ Species) + stat_smooth(method = "lm") + background_grid(major = 'y', minor = "none") + panel_border() plot.mpg <- ggplot(mpg, aes(x = cty, y = hwy, colour = factor(cyl))) + geom_point(size=2.5) g.iris <- ggplotGrob(plot.iris) # convert to gtable g.mpg <- ggplotGrob(plot.mpg) # convert to gtable iris.widths <- g.iris$widths[1:3] # extract the first three widths, # corresponding to left margin, y lab, and y axis mpg.widths <- g.mpg$widths[1:3] # same for mpg plot max.widths <- unit.pmax(iris.widths, mpg.widths) # calculate maximum widths g.iris$widths[1:3] <- max.widths # assign max. widths to iris gtable g.mpg$widths[1:3] <- max.widths # assign max widths to mpg gtable plot_grid(g.iris, g.mpg, labels = "AUTO", ncol = 1) The resulting plot is the following: 
What it should look like is this (with the y axis lines perfectly aligned vertically): 
The error seems to happen in this line:
g.iris$widths[1:3] <- max.widths Any insight would be appreciated.
Note that similar solutions have been used for a long time, see e.g. here. And the plot_grid() function in cowplot uses code like this also to align plots, and it still works. So this has me completely mystified.
