How do I customize the margin and label settings with plot_grid?

Question

I would like to have the title not be chopped off and have the axis labels removed from this chart that I generated with plot_grid from cowplot.

Here is my code

    data(mtcars)
 library(ggplot2)
 library(cowplot)
 mpg = ggplot() +
 geom_boxplot(aes(y = mpg,x = as.factor(cyl)),data=mtcars) +
 coord_flip()
 am=ggplot() +
 geom_boxplot(aes(y = mpg,x = as.factor(am)),data=mtcars) +
 coord_flip()
 vs=ggplot() +
 geom_boxplot(aes(y = mpg,x = as.factor(vs)),data=mtcars) +
 coord_flip()
 gear = ggplot() +
 geom_boxplot(aes(y = mpg,x = as.factor(gear)),data=mtcars) +
 coord_flip()
 p=plot_grid(mpg,am,vs,gear, labels = "Variables effecting Mileage", label_size = 14, hjust = -0.5,
 + vjust = 0.5)+theme_grey()
p

Also, if it would be simpler to create this without cowplot, what do you suggest?

Answer 1

Here is a cowplot only answer. It might be more what you want.

library(ggplot2)
library(cowplot)
library(gtable)

data(mtcars)

mpg = ggplot() +
  geom_boxplot(aes(y = mpg,x = as.factor(cyl)),data=mtcars) +
  coord_flip() + labs(x="",y="")
am=ggplot() +
  geom_boxplot(aes(y = mpg,x = as.factor(am)),data=mtcars) +
  coord_flip()+ labs(x="",y="")
vs=ggplot() +
  geom_boxplot(aes(y = mpg,x = as.factor(vs)),data=mtcars) +
  coord_flip()+ labs(x="",y="")
gear = ggplot() +
  geom_boxplot(aes(y = mpg,x = as.factor(gear)),data=mtcars) +
  coord_flip()+ labs(x="",y="")
p=plot_grid(mpg,am,vs,gear) +
  theme_grey() +

# Use annotation text as it is centered at the x,y point
  annotate("text",x=0.5,y=1.04,size=7,label="Variables affecting Mileage") +

# Add some space around the edges  
  theme(plot.margin = unit(c(1,0.5,0.5,0.5), "cm")) 


# Suppress tick marks
p=p+scale_y_continuous(breaks=NULL)+scale_x_continuous(breaks=NULL)

# Have to turn off clipping
gt <- ggplot_gtable(ggplot_build(p))
gt$layout$clip[gt$layout$name == "panel"] <- "off"

# need to draw it with the new clip settings
grid.draw(gt)

Yields: