showtext 包与数学公式表达式 expression 不兼容吗？

Cloud2016

当我运行 showtext::showtext_auto() 启用 showtext 包自动处理中文的能力后，运行如下代码块

library(ggplot2)
ggplot() +
  geom_function(
    fun = dgamma, args = list(shape = 9, scale = 1),
    aes(colour = "dgamma(x,9,1)"),
    linewidth = 1.2, xlim = c(0, 20)
  ) +
  geom_function(
    fun = dgamma, args = list(shape = 5, scale = 1),
    aes(colour = "dgamma(x,5,1)"),
    linewidth = 1.2, xlim = c(0, 20)
  ) +
  scale_colour_viridis_d(
    labels = c(
      "dgamma(x,9,1)" = expression(paste(alpha == 9, ", ", sigma == 1)),
      "dgamma(x,5,1)" = expression(paste(alpha == 5, ", ", sigma == 1))
    )
  ) +
  theme_bw()

会发现图例中的表达式集体向右挪了一步，见下图

如果不启用 showtext 包，表达式位置显示正常，见下图。

不知道是不是 showtext 包的 BUG？它是不是处理中文的同时对数学表达式不兼容，我不知道是什么原因？

运行信息

xfun::session_info(packages = c("ggplot2", "showtext"), dependencies = F)

R version 4.3.0 (2023-04-21)
Platform: x86_64-apple-darwin22.4.0 (64-bit)
Running under: macOS Ventura 13.3.1, RStudio 2023.3.0.386


Locale: en_US.UTF-8 / en_US.UTF-8 / en_US.UTF-8 / C / en_US.UTF-8 / en_US.UTF-8

time zone: Asia/Shanghai
tzcode source: internal

Package version:
  ggplot2_3.4.2  showtext_0.9-5

nan.xiao

我试着不用 showtext_auto() 而显式指定一个 font family，似乎就正常了：

library(showtext)
library(ggplot2)

font_add("PingFang SC", regular = "PingFang.ttc")

ggplot() +
  geom_function(
    fun = dgamma, args = list(shape = 9, scale = 1),
    aes(colour = "dgamma(x,9,1)"),
    linewidth = 1.2, xlim = c(0, 20),
  ) +
  geom_function(
    fun = dgamma, args = list(shape = 5, scale = 1),
    aes(colour = "dgamma(x,5,1)"),
    linewidth = 1.2, xlim = c(0, 20)
  ) +
  scale_colour_viridis_d(
    labels = c(
      "dgamma(x,9,1)" = expression(paste(alpha == 9, ", ", sigma == 1)),
      "dgamma(x,5,1)" = expression(paste(alpha == 5, ", ", sigma == 1))
    )
  ) +
  theme_bw(base_family = "PingFang SC") +
  xlab("中文测试")

我不懂原理，可能召唤原作者 @Ihavenothing 来回答一下比较好……

Cloud2016

nan.xiao 还真是，谢谢。

下面我把公式简化一下

ggplot() +
  geom_function(
    fun = dgamma, args = list(shape = 9, scale = 1),
    aes(colour = "list(alpha == 9, sigma == 1)"),
    linewidth = 1.2, xlim = c(0, 20), 
  ) +
  geom_function(
    fun = dgamma, args = list(shape = 5, scale = 1),
    aes(colour = "list(alpha == 5, sigma == 1)"),
    linewidth = 1.2, xlim = c(0, 20)
  ) +
  scale_colour_viridis_d(
    labels = scales::parse_format(),
    option = "C"
  )

Ihavenothing

nan.xiao 这个大概率是绘图设备直接用了系统的字体，因为不用 showtext_auto() 或者 showtext_begin() 的话 showtext 是不起作用的。

Ihavenothing

帮我去提个 issue 吧，就当是个备忘提醒，我可能要过段时间去看。