Annotating Facet Title as Strip Over Facet

Annotating facet title as strip over facet

Edit Updating to ggplot version 3.0.0

I think you were close. You had the row and column numbers wrong in the add grob command. Your new strip spans columns 5 to 9; that is, change 10 to 9 in the add grob command.

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

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(. ~ cyl)

z <- ggplotGrob(p)

#  New strip at the top
z <- gtable_add_rows(z, z$height[7], pos = 6)  # New row added below row 6

# Check the layout
gtable_show_layout(z)   # New strip goes into row 7 
                        # New strip spans columns 5 to 9

z <- gtable_add_grob(z, 
  list(rectGrob(gp = gpar(col = NA, fill = "gray85", size = .5)),
       textGrob("Number of Cylinders", gp = gpar(cex = .75, fontface = 'bold', col = "black"))), 
  t=7, l=5, b=7, r=9, name = c("a", "b"))

# Add small gap between strips - below row 6
z <- gtable_add_rows(z, unit(2/10, "line"), 7)

# Draw it
grid.newpage()
grid.draw(z)

OR, to add a new strip to the right:

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

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(cyl ~ .)

z <- ggplotGrob(p)

#  New strip to the right
z <- gtable_add_cols(z, z$widths[6], pos = 6)  # New column added to the right of column 6

# Check the layout
gtable_show_layout(z)   # New strip goes into column 7 
                        # New strip spans rows 7 to 11
z <- gtable_add_grob(z, 
  list(rectGrob(gp = gpar(col = NA, fill = "gray85", size = .5)),
       textGrob("Number of Cylinders", rot = -90,  
          gp = gpar(cex = .75, fontface = 'bold', col = "black"))), 
  t=7, l=7, b=11, r=7, name = c("a", "b"))

# Add small gap between strips - to the right of column 6
z <- gtable_add_cols(z, unit(2/10, "line"), 6)

# Draw it
grid.newpage()
grid.draw(z)

Annotating text on individual facet in ggplot2

Function annotate() adds the same label to all panels in a plot with facets. If the intention is to add different annotations to each panel, or annotations to only some panels, a geometry has to be used instead of annotate(). To use a geometry, such as geom_text() we need to assemble a data frame containing the text of the labels in one column and columns for the variables to be mapped to other aesthetics, as well as the variable(s) used for faceting.

Typically you'd do something like this:

ann_text <- data.frame(mpg = 15,wt = 5,lab = "Text",
                       cyl = factor(8,levels = c("4","6","8")))
p + geom_text(data = ann_text,label = "Text")

It should work without specifying the factor variable completely, but will probably throw some warnings:

Displaying annotation over facet_wrap label

One option would be the ggh4x package which offers some useful extensions of the default facets, e.g. using ggh4x::facet_wrap2 you could add you annotations as a second layer of facets. Additionally ggh4x::facet_wrap2 allows style the two layers separately via the strip argument:

df <- data.frame(x = rep(1:3, 4), mydat = rep(c("apple", "kiwi", "orange", "pear"), each = 3))
labels <- data.frame(
  mydat = as.factor(c("apple", "kiwi", "orange", "pear")),
  label = paste0("(", letters[1:4], ")")
)

df <- merge(df, labels, by = "mydat")

library(ggplot2)
library(ggh4x)

ggplot(df, aes(x, x)) +
  facet_wrap2(vars(label, mydat),
    strip = strip_themed(
      background_x = list(
        element_blank(),
        element_rect(fill = "grey85")
      ),
      text_x = list(
        element_text(hjust = 0),
        element_text()
      ),
      by_layer_x = TRUE
    )
  ) +
  geom_point() +
  coord_cartesian(clip = "off")

UPDATE As you mentioned in your comment you want the annotations as part of the strip label to save vertical space. IMHO the cleanest way to do this would be to add the annotations as part of the facet label. Just as an exercise in manipulating the gtable I wrote a custom function which allows to tag the facets with letters a, b, ...:

library(ggplot2)
library(grid)

g <- ggplot(df, aes(x, x)) +
  facet_wrap(~mydat) +
  geom_point() +
  coord_cartesian(clip = "off")

add_tag <- function(gg) {
  # Indices of strips in gtable
  ix_strips <- grep('strip', gg$layout$name)
  strips <- gg$grobs[ix_strips]
  # Strip positions
  strip_rc <- gsub("^.*?(\\d+)\\-(\\d+)$", "\\2-\\1", gg$layout$name[ix_strips])
  
  tag_order <- order(strip_rc)

  gg$grobs[ix_strips] <- mapply(function(x, y) {
    tag <- x$grobs[[1]]$children[[2]]
    tag$children[[1]]$label <- paste0("(", letters[[y]], ")")
    tag$children[[1]]$hjust <- 0
    tag$children[[1]]$x <- unit(0, "npc")
    
    x$grobs[[1]]$children[[2]] <- gList(
      tag, x$grobs[[1]]$children[[2]]   
    )
    
    x
  }, strips, tag_order)
  
  gg
}

gg = ggplotGrob(g)

gg <- add_tag(gg)

grid.draw(gg)

How to add annotation on each facet

library(ggplot2)

d <- data.frame(x=rep(1:3, 4), f=rep(letters[1:4], each=3))

labels <- data.frame(f=letters[1:4], label=LETTERS[1:4])
ggplot(d, aes(x,x)) +
  facet_wrap(~f) +
  geom_point() +
  geom_label(data = labels, aes(label=label), 
            x = Inf, y = -Inf, hjust=1, vjust=0,
            inherit.aes = FALSE)

Annotating individual facets in ggplot2

This will add labels to the min and max values of the confidence intervals:

p + geom_text(aes(label=min, x=min), vjust=1.5, size=4) +
  geom_text(aes(label=max, x=max), vjust=1.5, size=4)

This works because ggplot already has all the values it needs in the original data frame you passed to it. You just need to tell geom_text that you want the min and max values used as both the text labels and the x-axis locations placing them. ggplot takes care of the rest (i.e., the y-values and facet locations), because those are already an intrinsic part of the plot unless you override them.

Using annotate to add different annotations to different facets

With annotate, you can't. But by setting up a data.frame and using it as the data source for a geom_text, it is easy (with a few bookkeeping aspects).

d1 + geom_text(data=data.frame(x=0.25, y=1.5e+04, label=1:7, 
                               color=c("D","E","F","G","H","I","J")), 
               aes(x,y,label=label), inherit.aes=FALSE)

Using facet tags and strip labels together in ggplot2

Of course, I find a solution immediately after asking. The problem appears to be that tag_facet sets strip labels to element_blank, which can be fixed by calling theme after calling tag_facet.

# Load libraries
library(ggplot2)
library(egg)
#> Warning: package 'egg' was built under R version 3.5.3
#> Loading required package: gridExtra

# Create plot
p <- ggplot(mtcars, aes(mpg, wt)) 
p <- p + geom_point() 
p <- p + facet_grid(gear ~ cyl)
p <- tag_facet(p)
p <- p + theme(strip.text = element_text())
print(p)

^{Created on 2019-05-09 by the reprex package (v0.2.1)}

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