title: Subscript in multiple legends in ggplot2
body: <p>To make my figure suitable for black-white printing, I mapped one variable with "shape", "lty", "color" together.</p>
  
  <pre><code>ggplot(df, aes(x=time, y=mean, 
                 shape=quality, 
                 lty=quality,
                 color=quality))
  </code></pre>
  
  <p>I got the figure like,
  <a href="https://i.stack.imgur.com/bh99c.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bh99c.png" alt="enter image description here"></a>
  I would like to make part of legends as subscribs, with the codes:</p>
  
  <pre><code>labels=c(expression(Pol[(Art)]), expression(Pol['(Aca-)']), expression(Pol['(Aca-)']))
  </code></pre>
  
  <p>Unfortunately, when I put the "label" in color or shape, it makes the legend quite complex, like,</p>
  
  <p><a href="https://i.stack.imgur.com/5WJgi.png" rel="noreferrer"><img src="https://i.stack.imgur.com/5WJgi.png" alt="enter image description here"></a></p>
  
  <p>Is it possible to map "shape", "color","lty" to one varible, and set the subscript, but keep them in one set of legend?</p>