如何（更容易）在 ggplot2 中创建漂亮的 x 轴刻度（即 pi/2, pi, 3pi/2, ...）？

Question

I would like to create a plot, where one can see an alternative ticking of the x-axis, eg 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, eg 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)

Answer 2

Here is a function to make fractional labels (maybe a little clunky). It uses fractions from MASS package and allows you to change the multiplier you want to use on the x-axis. You just pass it a symbol (ie. "pi"). If the symbol has a value, the ticks will be scaled by width*value, otherwise just by width.

# Now plot the signal with the new x axis labels
p <- ggplot(data = df, aes(x = time, y = signal)) +
          geom_line()

## Convert x-ticks to fractional x-ticks with a symbol multiplier
fracAx <- function(p, symbol, width=0.5) {
    require(MASS)                                                 # for fractions
    val <- tryCatch(eval(parse(text=symbol)), error=function(e) 1)
    info <- ggplot_build(p)
    xrange <- info[[2]]$ranges[[1]]$x.range/val                   # get the x-range of figure
    vec.breaks <- seq(floor(xrange[1]), ceiling(xrange[2]), by=width)
    fracs <- strsplit(attr(fractions(vec.breaks), "fracs"), "/")  # convert to fractions
    labels <- sapply(fracs, function(i)
        if (length(i) > 1) { paste(i[1], "*", symbol, "/", i[2]) }
        else { paste(i, "*", symbol) })
    p + scale_x_continuous(breaks=vec.breaks*val, labels=parse(text=labels))
}
## Make the graph with pi axis
fracAx(p, "pi")

## Make the graph with e axis, for example
e <- exp(1)
fracAx(p, "e")

## Make the graph with a symbol that has no value
fracAx(p, "theta", width=2)

Answer 3

Based on the other answers here I was able to piece together some functions which implement a general radians format that can be used independently of mucking about with the internals of ggplot2 objects.

numerator <- function(x) {
  f = attr(x, "fracs")
  s <- as.integer(sign(x))
  ifelse(is.finite(x), as.integer(stringr::str_extract(f, "^[^/]*")), s)
}

denominator <- function(x) {
  f = attr(x, "fracs")
  s <- as.integer(sign(x))
  ratio <- str_detect(f, "/")
  d <- as.integer(stringr::str_extract(f, "[^/]*$"))
  ifelse(is.finite(x), ifelse(ratio, d, 1L), 0L)
}

char_sign <- function(x) {
  s <- sign(x)
  ifelse(s == 1, "+",
         ifelse(s == -1, "-", ""))
}

#' Convert value to radians formatting
radians <- function(x) {
  y = x/pi
  f = suppressWarnings(MASS::as.fractions(y))
  n = suppressWarnings(numerator(f))
  d = suppressWarnings(denominator(f))
  s <- char_sign(x)
  o <- vector(mode = "character", length = length(x))
  o <- ifelse(d == 0 & n != 0, paste0(s, "∞"), o)
  o <- ifelse(n == 0 & d != 0, "0", o)
  o <- ifelse(n != 0 & d != 0, paste0(n, "π/", d), o)
  o <- ifelse(n == 1 & d != 0, paste0("π/", d), o)
  o <- ifelse(n == -1 & d == 1, paste0(s, "π"), o)
  o <- ifelse(n == -1 & d != 0 & d != 1, paste0(s, "π/", d), o)
  o <- ifelse(d == 1 & n != 0 & abs(n) != 1, paste0(n, "π"), o)
  o <- ifelse(n == d & is.finite(n), "π", o)
  o
}

Here it is in use:

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

ggplot(df, aes(time, signal)) +
  geom_line() +
  scale_x_continuous(labels = trans_format(radians, force),
                     breaks = seq(0, 4*pi, pi/2))