如何为ggplot2柱状图标题中的数字6添加下标？

需要为ggplot2柱状图标题“PSII monomer, Cyt b6f”里的数字“6”添加下标，尝试使用expression、bquote等函数但未成功，相关代码如下：

ggplot(data=summary_Lane3, mapping = aes(x = factor(time), y = rd, fill= gc))+
  geom_bar(stat = "identity", position = "dodge")+
  labs(title=expression("PSII monomer, Cyt b[6]f"),x= "Time (hr)",  y="Relative density", fill= "Growth conditions")+
  ylim(0,2.1)+
  geom_errorbar(aes(ymin=rd-se, ymax=rd+se), width=.5, position=position_dodge(width= 0.9))+
  stat_compare_means(data=Lane3, label="p.signif", label.y= 1.85, method="anova")+
  theme_bw()+
  theme(text = element_text(size = 15))

你之前的错误在于给expression套了外层引号，导致整个文本被当作普通字符串，R不会解析里面的下标语法[]。以下是几种可行的修正方案：

方案1：直接在expression中组合元素 #

用~来添加空格，无需外层引号：

labs(title=expression(PSII~monomer~","~Cyt~b[6]f), x= "Time (hr)",  y="Relative density", fill= "Growth conditions")

方案2：用paste拼接文本（更直观） #

适合需要混合普通文本和表达式的场景：

labs(title=expression(paste("PSII monomer, Cyt ", b[6], "f")), x= "Time (hr)",  y="Relative density", fill= "Growth conditions")

方案3：用bquote（支持动态变量） #

如果后续需要动态修改下标数值，bquote会更灵活，当前场景下写法如下：

labs(title=bquote(PSII~monomer~","~Cyt~b[.(6)]f), x= "Time (hr)",  y="Relative density", fill= "Growth conditions")

ggplot(data=summary_Lane3, mapping = aes(x = factor(time), y = rd, fill= gc))+
  geom_bar(stat = "identity", position = "dodge")+
  labs(title=expression(paste("PSII monomer, Cyt ", b[6], "f")),x= "Time (hr)",  y="Relative density", fill= "Growth conditions")+
  ylim(0,2.1)+
  geom_errorbar(aes(ymin=rd-se, ymax=rd+se), width=.5, position=position_dodge(width= 0.9))+
  stat_compare_means(data=Lane3, label="p.signif", label.y= 1.85, method="anova")+
  theme_bw()+
  theme(text = element_text(size = 15))

内容的提问来源于stack exchange，提问作者wychin