Change ggplot2 colourbar tick marks to black

Question

In some of my plots I find it hard to see the tick marks in the colour bar. I haven't been able to find a documented way to change the colour of the ticks. All the examples seem to focus on changing the labels or not drawing ticks at all. Is it possible?

#  Data
require(ggplot2)
require(grid)
n <- 100
x <- y <- seq(-4*pi, 4*pi, len=n)
r <- cos( sqrt( outer(x^2, y^2, "+") ) ^ 2 )
df <- data.frame( x = rep( x , each = n) , y = rep( y , times = n ) , val = c(r) )

#  Plot
ggplot( df , aes( x , y , fill = val ) )+
  geom_raster()+
  scale_fill_gradient( low = "#FFFFFF" , high = "#de2d26" )+
  guides( fill = guide_colourbar( barheight = unit( 3 , "in" ) ) )+
  theme_bw()+
  theme( line = element_line( colour = "#0000FF" ) )

How can I make the ticks in the colourbar be plotted in black rather than white, without changing other elements of the plot?

---

p.s. kudos to this question for the function to create the example data

Answer 1

The pull request mentioned in another answer never made it into the ggplot2 code base, but this is now possible in a slightly different way in the development version (slated to be released as ggplot2 2.3):

ggplot(df, aes(x, y, fill = val)) +
  geom_raster() +
  scale_fill_gradient(low = "#FFFFFF", high = "#de2d26") +
  guides(fill = guide_colourbar(barheight = unit( 3 , "in" ),
                                ticks.colour = "black",
                                ticks.linewidth = 1)) +
  theme_bw() +
  theme(line = element_line(colour = "#0000FF"))

You can also add a frame, which may be useful when some of the colors in the colorbar are very light.

ggplot(df, aes(x, y, fill = val)) +
  geom_raster() +
  scale_fill_gradient(low = "#FFFFFF", high = "#de2d26") +
  guides(fill = guide_colourbar(barheight = unit( 3 , "in" ),
                                ticks.colour = "black",
                                ticks.linewidth = 1,
                                frame.colour = "black",
                                frame.linewidth = 1)) +
  theme_bw() +
  theme(line = element_line(colour = "#0000FF"))

Answer 2

EDIT: Please refer to the answer below by Claus Wilke.

---

I have included the original answer below, but please note that it is now outdated and I do not recommend using it.

I included the functionality to customize tick marks and legend borders in my fork of ggplot2. I have submitted a pull request, but thought I would let people know here in case they stumble on this question.

Install my fork using the following code,

if (!require(devtools))
  install.packages('devtools')
install_github('paleo13/ggplot2')

We can specify black marks using the following code,

ggplot(df, aes( x, y, fill = val)) +
  geom_raster() +
  scale_fill_gradient(low = "#FFFFFF", high = "#de2d26") +
  theme_bw() +
  theme(line = element_line(colour = "#0000FF")) +
  guides(fill = guide_colourbar(barheight = unit(3, "in"),
   ticks=element_line(color='black'), border=element_line(color='black')))

(I would have included this as a comment but I lack the reputation to do so, if anyone with sufficient privileges wants to delete this answer and move the contents to the comments then feel free)