ggplot2 & facet_wrap - eliminate vertical distance between facets

Question

I'm working with some data that I want to display as a nxn grid of plots. Edit: To be more clear, there's 21 categories in my data. I want to facet by category, and have those 21 plots in a 5 x 5 square grid (where the orphan is by itself on the fifth row). Thus facet_wrap instead of facet_grid.

I've got the following code written up for doing it (using the good old iris data set for my reproducible example):

library(ggplot2)
library(grid)

cust_theme <- theme_bw() + theme(legend.position="none", 
              axis.title = element_blank(), axis.ticks = element_blank(), 
              axis.text = element_blank(), strip.text = element_blank(), 
              strip.background = element_blank(), panel.margin = unit(0, "lines"), 
              panel.border = element_rect(size = 0.25, color = "black"), 
              panel.grid = element_blank())

iris.plot <- ggplot(data = iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
             geom_point() + cust_theme + facet_wrap( ~ Species, ncol = 2) + 
             labs(title = "Irises by species")

This gives me ALMOST what I want, but not quite:

I've still got a tiny strip of space between the top row of plots and the bottom row. I'd like to get rid of that entirely, but panel.margin is obviously not doing it. Is there a way to do this?

Answer 1

This might be a little late, but panel.marginis now deprecated. Inside theme use panel.spacing. To eliminate the spacing between the facets then load the grid package and use panel.spacing = unit(0, "lines")