How can one (easier) create nice x-axis ticks (i.e. pi/2, pi, 3pi/2, ...) in ggplot2?

Question

I would like to create a plot, where one can see an alternative ticking of the x-axis, e.g. pi/2, pi, 3pi/2, etc. So far, this works for me only with a rather unhandy code (look at the lines where I create pi.halfs, pi.fulls and merge them later into vec.expr):

require (ggplot2)

# Create vectors: breaks and corresponding labels as multiples of pi/2
vec.breaks <- seq(from = pi/2, to = 7*pi/2, by = pi/2)
pi.halfs <- c(paste(expression(pi), "/2"),
  paste(seq(from = 3, to = 21, by = 2), "*" , expression(pi), "/2"))
pi.fulls <- c(paste(expression(pi)),
  paste(seq(from = 2, to = 11, by = 1), "*" , expression(pi)))
vec.expr <- parse(text = c(rbind(pi.halfs, pi.fulls)))[1:7]

# Create some time and signal
time <- seq(from = 0, to = 4*pi, by = 0.01)
signal <- sin(time)
df <- data.frame(time,signal)

# Now plot the signal with the new x axis labels
fig <- ggplot(data = df, aes(x = time, y = signal)) +
          geom_line() +
          scale_x_continuous(breaks=vec.breaks, labels=vec.expr)
print(fig)

... resulting in ...

Is anyone aware of an easier approach, where one can change the base of some x-axis labeling in ggplot2, e.g. like here from decimals to multiples of pi? Are there any nice packages, that I missed so far? I found some duplicates of this question, but only in other languages...

Answer 1

You are looking for the scales package, which lets you create arbitrary formatting functions for scales and also has a number of helpful formatting functions already built in. Looking through the scales package help, I was surprised not to find a radian scale, but you should be able to create one using math_formatter(). This code gets the same results, though not with the fractions.

library(ggplot2)
library(scales)

time <- seq(from = 0, to = 4*pi, by = 0.01)
signal <- sin(time)
df <- data.frame(time,signal)

pi_scales <- math_format(.x * pi, format = function(x) x / pi)

fig <- ggplot(data = df, aes(x = time, y = signal)) +
  geom_line() +
  scale_x_continuous(labels = pi_scales, breaks = seq(pi / 2, 7 * pi / 2, pi / 2))

print(fig)