Add secondary X axis labels to ggplot with one X axis

Question

**Edit, there are two great solutions here, one is marked as the answer, but @hrbrmstr provides a great solution combining two ggplots which works well for this simple plot.*

Here's the code

breaks.major <- c(0,15,37.5,52.5,67.5,82.5,95,100) #defines the midpoints of the categories (label locations)
breaks.minor <- c(30,45,60,75,90) #defines the edges of the categories (second label set I need)
labels.minor <- c("","Extremely \nDissatisfied","Dissatisfied","Uncertain","Satisfied","Very \nSatisfied","Extremely \nSatisfied","")
lims =c(0,100)
g <- ggplot(mpg, aes(class))+
  geom_bar()+
  coord_flip()+
  scale_y_continuous(limit = lims, minor_breaks = breaks.minor, breaks = breaks.major, labels = labels.minor) +
  theme(panel.grid.major.x = element_blank()) +
  theme(panel.grid.major.y = element_blank()) +
  theme(axis.ticks.x=element_blank()) +
  theme(axis.title= element_blank()) 

It produces this plot:

I need to have two sets of X-axis labels, one showing the category names (i.e. the "satisfied" etc. that are already there via labels.minor), and one showing the values at the breaks.minor locations (corresponding to the category limits, i.e. the vertical panel grid lines). I need the current labels.minor labels to be below the required additional labels.

I currently do this with line breaks so that the numbers and categories are all in one long string, but the spacing gets funny with plot resizes.I could do this with text boxes (I assume), is there a way within ggplot?

Extra points if you get my current labels in the centre of their sections (e.g. "Extremely Satisfied" is off-centre)

This is my desired output (pardon my 'mspaint')

Answer 1

I think this does what you're looking for:

library(ggplot2)
library(grid)
library(gtable)
library(gridExtra)

breaks.major <- c(0, 15, 37.5, 52.5, 67.5, 82.5, 95, 100)
breaks.minor <- c(30, 45, 60, 75, 90)
labels.minor <- c("", "Extremely\nDissatisfied", "Dissatisfied", "Uncertain",
                  "Satisfied", "Very\nSatisfied", "Extremely\nSatisfied", "")
lims <- c(0, 100)

# build the main plot with the text axis
gg1 <- ggplot(mpg, aes(class))
gg1 <- gg1 + geom_bar()
gg1 <- gg1 + scale_y_continuous(expand=c(0,0), limit=lims, 
                                minor_breaks=breaks.minor, 
                                breaks=breaks.major,
                                labels=labels.minor)
gg1 <- gg1 + coord_flip()
gg1 <- gg1 + theme(panel.grid.major.x=element_blank())
gg1 <- gg1 + theme(panel.grid.major.y=element_blank())
gg1 <- gg1 + theme(axis.ticks.x=element_blank())
gg1 <- gg1 + theme(axis.title=element_blank())

# let ggplot2 do the work of building the second axis
gg2 <- ggplot(mpg, aes(class))
gg2 <- gg2 + scale_y_continuous(expand=c(0,0), limit=lims, 
                                breaks=c(0, breaks.minor, 100))
gg2 <- gg2 + coord_flip()
gg2 <- gg2 + theme(axis.ticks.x=element_blank())
gg2 <- gg2 + theme(axis.text.x=element_text(hjust=c(0, 0.5, 0.5, 0.5, 0.5, 0.5, 1)))

gt1 <- ggplot_gtable(ggplot_build(gg1))
gt2 <- ggplot_gtable(ggplot_build(gg2))
axis2 <- grid.arrange(gt2$grobs[[5]])

gt <- gtable_add_rows(gt1, unit(0.1, "null"), 4)
grid.arrange(gtable_add_grob(gt, axis2, t=5, l=4, b=5, r=4))