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

Question

我想创建一个绘图,其中可以看到x轴的替代滴答,例如pi/2,pi,3pi/2等.到目前为止,这对我来说只适用于一个相当不方便的代码(看看在那里我创建的线条pi.halfs,pi.fulls后来将它们合并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)

... 导致 ...

是否有人知道一种更简单的方法,可以在ggplot2中更改某些x轴标记的基础,例如像这里从小数到pi的倍数？到目前为止我有没有错过的套餐？我发现了这个问题的一些重复,但仅限于其他语言......

Answer 1

您正在寻找缩放包,它允许您为缩放创建任意格式化功能,并且还内置了许多有用的格式化功能.通过缩放包帮助,我很惊讶没有找到弧度,但你应该能够创建一个使用math_formatter().此代码获得相同的结果,但不是分数.

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

这是一个制作分数标签的函数（可能有点笨拙）。它使用包fractions中的MASS内容并允许您更改要在 x 轴上使用的乘数。您只需向其传递一个符号（即“pi”）。如果符号有值，则刻度将按宽度*值缩放，否则仅按宽度缩放。

# 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)