title: How to label panels in lattice
body: <p>here is a simple problem that surely you already come across, but is giving me a big headache...</p>
  
  <p>I have a dataframe like this:</p>
  
  <pre><code>set.seed(3)
  mydata &lt;- data.frame(var = rnorm(100,20,1),
                       temp = sin(sort(rep(c(1:10),10))),
                       subj = as.factor(rep(c(1:10),5)))
  </code></pre>
  
  <p>and I need to make a scatter plot for each subj, not a problem, but...
  what i want is to replace the strips from the lattice and add a label to each plot.
  I manage to do this with the following code, but I'm still not happy...</p>
  
  <pre><code>xyplot(var ~ temp | subj, 
         data = mydata,
         strip=FALSE,
         panel = function(x, y,...) {
                 panel.xyplot(x, y,...)
                 panel.text(1,21,labels=which.packet())
                 }) 
  </code></pre>
  
  <p>The last bit...where i got stacked...is how to print letters instead of numbers in each panel. I would like to call the panels a,b,c... and so on.</p>
  
  <p>Any suggestions...
  Many thanks
  matteo </p>