r - 如何在我的一个方面标签中使用希腊符号？

Question

我正在尝试使用 ggplot2 (v. 2_2.2.1) facet_wrap 创建一个绘图，我需要在一个方面标签(五个方面)中只包含一个希腊符号。我尝试使用在 Stack Overflow 上发布的代码:

• R Greek letters and normal letters for facet label in R facet grid
• How to add expressions to labels in facet_wrap?
• How can I use grid to edit a ggplot2 object to add math expressions to facet labels?

但没有任何成功(getGrob 不工作，mf_labeller 也是如此)。

有人可以帮我吗？

示例代码如下:

df <- data.frame(genes = rep(c("BA","MLL","pos","neg","PMLalpha+"),5), value = sample(1:100, 25, replace=TRUE))
df$genes <- factor(df$genes, levels = c("BA","MLL","pos","neg","PMLalpha+"), ordered = TRUE)
ggplot(df,aes(x=genes, y=value)) + geom_boxplot()+ 
  facet_wrap(~genes, ncol = 5,scales = "free_x")+
  scale_x_discrete(name="",
               breaks = c("BA","MLL","pos","neg","PMLalpha+"),
               labels = c("BA","MLL","pos","neg",expression(paste("PML", alpha,"+"))))

我可以重命名 x 轴刻度，但我不能在 facet 标签内这样做。

Answer 1

这是一个使用 labeller=label_parsed 的技巧。

首先，为使用要解析的表达式的df$genes 定义标签:

df$genes <- factor(df$genes, levels = c("BA","MLL","pos","neg","PMLalpha+"),
 ordered = TRUE, labels=c("BA","MLL","pos","neg",expression(paste("PML", alpha,"+"))))

然后在facet_wrap中使用labeller=label_parsed:

ggplot(df,aes(x=genes, y=value)) + geom_boxplot() + 
  facet_wrap(~genes, ncol = 5, scales = "free_x", labeller = label_parsed)

如您所见，这会弄乱 x 轴标签，但您可以在 scale_x_discrete 中修复它，如下所示:

ggplot(df,aes(x=genes, y=value)) + geom_boxplot() + 
  facet_wrap(~genes, ncol = 5, scales = "free_x", labeller = label_parsed) +
  scale_x_discrete(name="",
               breaks = c("BA","MLL","pos","neg","paste(\"PML\", alpha, \"+\")"),
               labels = c("BA","MLL","pos","neg", expression(paste("PML", alpha,"+"))))

编辑:作为对发布的后续问题的回应，这里是对多个变量使用 label_parsed 的解决方案。在变量字符串中包含请求的空间。

df$var2 <- c(rep(c('trt a','trt b'), NROW(df)/2),'trt a')
df$var2 <- factor(df$var2, levels=c('trt a','trt b'), ordered=T, labels=c(expression(paste("trt ", alpha)), expression(paste("trt ", beta))))

ggplot(df,aes(x=var2, y=value)) + geom_boxplot() +facet_wrap(~var2, ncol = 5, scales = "free_x", labeller = label_parsed)

固定x轴:

ggplot(df,aes(x=var2, y=value)) + geom_boxplot() + 
  facet_wrap(~var2, ncol = 5, scales = "free_x", labeller = label_parsed) + 
  scale_x_discrete(name="",
                 breaks = c(expression(paste("trt ", alpha)), expression(paste("trt ", beta))),
                 labels = c(expression(paste("trt ", alpha)), expression(paste("trt ", beta))))