title: x-axis label in seqdplot
body: <p>I have problems labelling my x-axis when I nplot the state-distribution by time points in sequence analysis using seqdplot() from the package TraMinR. The labelling works with other plotting functions like <code>seqiplot</code>. I can label the x-axis later with <code>title(xlab="")</code> however I don't know how this works when my sequences are grouped e.g. by gender. Am I missing something what makes the seqdplot function special? Thanks for your help! </p>
  
  <p>Here is an example</p>
  
  <pre><code>library(TraMineR)
  
  data(mvad)
  mvad.labels &lt;- c("employment", "further education", "higher education",
                   "joblessness", "school", "training")
  mvad.scodes &lt;- c("EM","FE","HE","JL","SC","TR")
  mvad.seq &lt;- seqdef(mvad, 15:86, states=mvad.scodes, labels=mvad.labels)
  
  # Index plot: labelling works
  seqiplot(mvad.seq, with.legend=F, xlab="time")
  
  # State Distribution Plot: labelling doesn't work
  seqdplot(mvad.seq, with.legend=F, xlab="time") 
  </code></pre>