title: xlab in theme in ggplot2
body: <p>I'm making many graphs which have same xlab. I wonder can we use xlab in theme like this:</p>
  
  <pre><code>theme_mine1 &lt;- opts(
        panel.grid.major = theme_blank()
      , panel.grid.minor = theme_blank()
      , panel.background = theme_blank()
      , axis.line        = theme_blank()
      , xlab             = expression(paste("My (xlab ", m^{-2}, ")"))
      )
  </code></pre>
  
  <p>When I use this theme it doesn't give any error or warning but it does not change the xlab. Any suggestion will be highly appreciated. Thanks</p>