title: apply the same mtext to every page
body: <p>I have multiple plots across multiple pages, how do I provide the same mtext for all page? below is an example data-set:</p>
  
  <pre><code>bootData &lt;- lapply(seq(1,24), function(x) rnorm(50))
  names(bootData) &lt;- c("0", "2", "4", "6", "8", "10", "15", "20", "30", "40", "45", 
  "50", "55", "60", "65", "70", "80", "85", "90", "92", "94", "96", 
  "98", "100")
  </code></pre>
  
  <p>Plot the data</p>
  
  <pre><code>pdf(file = "test.pdf", width = 11.5, height = 8.5)
  par(mar=c(5,4,3,2), oma=c(3,3,1,3), mfrow = c(2,3))
  lapply(names(bootData), function(x) hist(bootData[[x]], main = paste("Histogram of", x)))
  
  mtext("Error per label", side = 1, line = 1, outer = TRUE,  col = "firebrick")
  dev.off()
  </code></pre>
  
  <p>I would like the mtext "Error per label" to appear in every page. With the above code it only appears in the last page.</p>
  
  <p>Many thanks in advance.</p>