Changing the Appearance of Facet Labels size

Question

I know the question was asked here: Is there a way to increase the height of the strip.text bar in a facet?

I want to decrease the height of the strip.text bar without changing the text size. In the current case there is always a space left between text and strip bar walls.

Here is what I tried so far,

library(gcookbook) # For the data set
library(ggplot2)

ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
facet_grid(.~ Date) +
theme(strip.text = element_text(face="bold", size=9,lineheight=5.0),
strip.background = element_rect(fill="lightblue", colour="black",
size=1))

In my case it seems that lineheight does not affect anything even if changed to 5. Why?
How can I make the strip bar size a little smaller but keeping the text size the same?

edit after @Sandy Muspratt answer

we are able to reduce the strip size if there is only one row of facets.

g = ggplotGrob(p)
g$heights[c(3)] = unit(.4, "cm")  # Set the height

grid.newpage()
grid.draw(g)

However, in my real data I have many rows of plot like below and when I changed the elements of g$heights nothing happened!

p = ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
  facet_wrap(~ Date,ncol = 1) +
  theme(strip.text = element_text(face="bold", size=9),
        strip.background = element_rect(fill="lightblue", colour="black",size=1))

 g = ggplotGrob(p)
g$heights
#    [1] 5.5pt               0cm                 0.66882800608828cm  #1null               0cm                 0.193302891933029cm
#     [7] 0.66882800608828cm  1null               0cm                 #0.193302891933029cm 0.66882800608828cm  1null              
#    [13] 0.456194824961948cm 0cm                 1grobheight         5.5pt

then I attempted to change 1,7 and 11 elements

g$heights[c(3,7,11)] = unit(.4, "cm")  # Set the height

grid.newpage()
grid.draw(g)

No change in the facet label size.

> g$heights
 [1] 5.5pt                                                       1grobheight                                                
 [3] sum(0.2cm, sum(0.15cm, 0.8128cm, 0cm, 0.15cm), 0.2cm)+0.2cm 0.2                                                        
 [5] 1null                                                       0cm                                                        
 [7] 0.193302891933029cm                                         0.2                                                        
 [9] 1null                                                       0cm                                                        
[11] 0.193302891933029cm                                         0.2                                                        
[13] 1null                                                       0cm                                                        
[15] 0.193302891933029cm                                         0.2                                                        
[17] 1null                                                       0.456194824961948cm                                        
[19] 0cm                                                         1grobheight                                                
[21] 5.5pt  

Answer 1

Use margins

From about ggplot2 ver 2.1.0: In theme, specify margins in the strip_text element (see here).

library(ggplot2)
library(gcookbook) # For the data set

p = ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
facet_grid(. ~ Date) +
theme(strip.text = element_text(face="bold", size=9),
strip.background = element_rect(fill="lightblue", colour="black",size=1))

  p +
  theme(strip.text.x = element_text(margin = margin(.1, 0, .1, 0, "cm")))

The original answer updated to ggplot2 v2.2.0

Your facet_grid chart

This will reduce the height of the strip (all the way to zero height if you want). The height needs to be set for one strip and three grobs. This will work with your specific facet_grid example.

library(ggplot2)
library(grid)
library(gtable)
library(gcookbook) # For the data set

p = ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
facet_grid(. ~ Date) +
theme(strip.text = element_text(face="bold", size=9),
strip.background = element_rect(fill="lightblue", colour="black",size=1))

g = ggplotGrob(p)

g$heights[6] = unit(0.4, "cm")  # Set the height

for(i in 13:15) g$grobs[[i]]$heights = unit(1, "npc") # Set height of grobs

grid.newpage()
grid.draw(g)

Your Facet_wrap chart

There are three strips down the page. Therefore, there are three strip heights to be changed, and the three grob heights to be changed.

The following will work with your specific facet_wrap example.

p = ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
  facet_wrap(~ Date,ncol = 1) +
  theme(strip.text = element_text(face="bold", size=9),
        strip.background = element_rect(fill="lightblue", colour="black",size=1))

g = ggplotGrob(p)

for(i in c(6,11,16)) g$heights[[i]] = unit(0.4,"cm")   # Three strip heights changed
for(i in c(17,18,19)) g$grobs[[i]]$heights <-  unit(1, "npc")   # The height of three grobs changed

grid.newpage()
grid.draw(g)

How to find the relevant heights and grobs?

g$heights returns a vector of heights. The 1null heights are the plot panels. The strip heights are one before - that is 6, 11, 16.

g$layout returns a data frame with the names of the grobs in the last column. The grobs that need their heights changed are those with names beginning with "strip". They are in rows 17, 18, 19.

To generalise a little

p = ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
  facet_wrap(~ Date,ncol = 1) +
  theme(strip.text = element_text(face="bold", size=9),
        strip.background = element_rect(fill="lightblue", colour="black",size=1))

g = ggplotGrob(p)

# The heights that need changing are in positions one less than the plot panels
pos =  c(subset(g$layout, grepl("panel", g$layout$name), select = t))
for(i in pos) g$heights[i-1] = unit(0.4,"cm")

# The grobs that need their heights changed:
grobs = which(grepl("strip", g$layout$name))
for(i in grobs) g$grobs[[i]]$heights <-  unit(1, "npc")      
grid.newpage()
grid.draw(g)

Multiple panels per row

Nearly the same code can be used, even with a title and a legend positioned on top. There is a change in the calculation of pos, but even without that change, the code runs.

library(ggplot2)
library(grid)

# Some data
df = data.frame(x= rnorm(100), y = rnorm(100), z = sample(1:12, 100, T), col = sample(c("a","b"), 100, T))

# The plot
p = ggplot(df, aes(x = x, y = y, colour = col)) +
   geom_point() +
   labs(title = "Made-up data") + 
   facet_wrap(~ z, nrow = 4) +
   theme(legend.position = "top")

g = ggplotGrob(p)

# The heights that need changing are in positions one less than the plot panels
pos =  c(unique(subset(g$layout, grepl("panel", g$layout$name), select = t)))
for(i in pos) g$heights[i-1] = unit(0.2, "cm")

# The grobs that need their heights changed:
grobs = which(grepl("strip", g$layout$name))
for(i in grobs) g$grobs[[i]]$heights <-  unit(1, "npc") 

grid.newpage()
grid.draw(g)