cbind on ggplotGrob objects fails with "Error in mmm < each : comparison of these types is not implemented"

Question

This appears to be similar to an issue that @baptiste was trying to work around back in 2014. I am revisiting code I wrote back in June that involved creating three ggplotGrobs and combining them with a call to cbind. Now, this code fails with the message: "Error in mmm < each : comparison of these types is not implemented."

I thought that this would be specific to my particular application, but I was able to make a very simple, reproducible example. Even if the cbind is performed on two identical ggplotGrobs, this code still fails.

library(ggplot2)
library(gtable)

# Make some plots
pl1 <- ggplot(mtcars, aes(x = disp, y = mpg)) +
    geom_point()
pl2 <- ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, colour = Species)) +
    geom_point()

# Convert to grobs
pl1_grob <- ggplotGrob(pl1)
pl2_grob <- ggplotGrob(pl2)

# Bind them together -- Error!
combined_grob <- cbind(pl1_grob, pl2_grob)

The error and relevant traceback are here:

> combined_grob <- cbind(pl1_grob, pl2_grob)
Error in mmm < each : comparison of these types is not implemented
> traceback()
8: comp(x_val, y_val)
7: unit(comp(x_val, y_val), x_unit)
6: compare_unit(x$heights, y$heights, pmax)
5: cbind_gtable(x, y, size = size)
4: f(init, x[[i]])
3: Reduce(function(x, y) cbind_gtable(x, y, size = size), gtables)
2: cbind(deparse.level, ...)
1: cbind(pl1_grob, pl2_grob)

This code fails with R-3.4.2 on OS X 10.11.6 with grid_3.4.2, gtable_0.2.0, and ggplot2_2.2.1, AND with R-3.3.2 on Linux (compiled from source on Ubuntu 16.04) with grid_3.3.2, gtable_0.2.0, and ggplot2_2.2.1.

Before making the example above, I noticed that one of the vignettes in the "lemon" package ("gtable_show_lemonade") fails near the very end with the same error upon a cbind of two gtables. I confirmed this error by running the code for this vignette from source, which goes something like:

library(lemon)
edit(vignette('gtable_show_lemonade', package = 'lemon'))
# Then use whichever editor you opened to copy the temporary filename
# of the vignette source, and run this with source()

The compiled vignette (with failed cbind) is here: https://cran.r-project.org/web/packages/lemon/vignettes/gtable_show_lemonade.html

The failure during vignette compilation was obtained using the aforementioned R-3.4.2 instance running on OS X 10.11 (El Capitan).

I would appreciate any help with working around this! Since I would like to use my Mac to generate the figures (better font situation), I'm hoping there is a way I can overload a function to fix this.

UPDATE:

This is "only" a problem if size = 'max', which is the default, is specified (or size = 'min'). I think that I can work around it using size = 'first' or size = 'last', which do not perform the offending height comparisons, but it is certainly not convenient that the default behavior doesn't work (and there are certain situations size = 'max' is useful).

Answer 1

I never managed to get this fixed in gtable so gridExtra has a modified version called gtable_cbind.