Alter just horizontal spacing between facets (ggplot2)

As of July 9th, 2015, the panel.margin.x and panel.margin.y seem to have been implemented

p <- p + theme(panel.margin.x=unit(0.5, "lines") , panel.margin.y=unit(1,"lines"))

As of December 15, 2016, 'panel.spacing' and 'panel.spacing.x' is implemented in r 3.3.2 and ggplot2 2.2.0

p <- p + theme(panel.spacing.x=unit(0.5, "lines"),panel.spacing.y=unit(1, "lines"))

A manual solution until this feature becomes available:

library(grid)
height <- 0.5 # Vertical spacing
aux <- 1e-5 # Auxiliary number to identify 'height' among other heights
width <- 0.1 # Desirable horizontal spacing

p <- p + theme(panel.margin = unit(height + aux, "lines"))

gtable <- ggplot_gtable(ggplot_build(p))
gtable$widths[sapply(gtable$widths, '[[', 1) == height + aux][[1]][[1]] <- width
grid.draw(gtable)