title: Plotting axis labels with Greek symbols from a vector
body: <p>I am struggling trying to plot Greek symbols as axis labels. Due to for looping several plots with different scales, I would like to use a vector that contains the symbol name instead of using the symbol name within <code>expression()</code>, which works fine:</p>
  
  <pre><code>x &lt;- rnorm(10, 5,6)
  y &lt;- rnorm(10, 2,1)
  xlab &lt;- expression(paste(mu, "mol/mol"))
  plot(x,y, xlab = xlab)
  </code></pre>
  
  <p>For some reason, I can't replace "mu" with a vector containing the symbol name:</p>
  
  <pre><code>k &lt;- "mu"
  xlab &lt;- expression(paste(get(k), "mol/mol"))
  plot(x,y, xlab = xlab)
  </code></pre>
  
  <p>I found a similar <a href="https://stackoverflow.com/questions/4973898/combining-paste-and-expression-functions-in-plot-labels">question</a>, but what worked for functions does not seem to work for Greek symbols:</p>
  
  <pre><code>ylab &lt;- substitute(paste(nn, "mol/mol"), list(nn=k))
  xlab &lt;- bquote(.(k) * "mol/mol")
  
  plot(x,y, ylab = ylab, xlab = xlab)
  </code></pre>
  
  <p>What do I do wrong? How to get a Greek symbol to axis label through a vector?</p>