title: how to subscript the x axis tick label
body: <p>I generated this graph with the below script. <img src="https://i.stack.imgur.com/DEDRc.png" alt="enter image description here">
  But how could I subscript the "10" in PM10, "2" in SO2, and"2" in NO2?</p>
  
  <p>I tried <code>levels(df$variable) &lt;- c("PM[10]","SO[2]", "NO", "NO[2]")</code>, but does not work.</p>
  
  <p>Can anyone help? Thank you!</p>
  
  <pre><code>variable &lt;- c("PM10","SO2","NO","NO2")
  coef &lt;- c(10,20,30,40)
  coef_lb &lt;- c(-5,10,23,27)
  coef_ub &lt;- c(20,39,39,50)
  
  df &lt;- as.data.frame(cbind(variable, as.numeric(coef),as.numeric(coef_lb),as.numeric(coef_ub)))
  
  
  df$variable &lt;- factor(df$variable,levels=c("PM10","SO2","NO","NO2"))
  levels(df$variable) &lt;- c("PM[10]","SO[2]", "NO", "NO[2]")
  
  library(ggplot2)
  #ggplot 95%CI
  BWplot &lt;- ggplot(data=df,aes(x=variable,y=coef))
  BWplot &lt;- BWplot + geom_pointrange(aes(ymin=coef_lb,ymax=coef_ub))
  BWplot &lt;- BWplot + geom_point() 
  BWplot &lt;- BWplot + scale_y_continuous(limits=c(-110, 110),breaks=seq(-100, 100, by = 20))
  BWplot &lt;- BWplot + xlab("Air pollutant") 
  BWplot &lt;- BWplot + ylab("Mean change") 
  BWplot &lt;- BWplot + geom_hline(yintercept=0,alpha=0.5)
  BWplot
  </code></pre>