title: How to set custom tick interval and labels on date x.axis on standard plot
body: <p>I'm working on an animal tracking time series dataset.</p>
  
  <p>I'm trying to plot the date (x axis) on top of the graph (side=3) instead of below as automatically done by R with the plot function. However, I would like the tick intervals to be similar to the below axis (or ideally 7-day intervals) instead of showing everyday ticks like it currently does on the top axis. Any suggestion on how to achieve this?</p>
  
  <p>This is probably trivial... Sorry if the answer is obvious to some of you.</p>
  
  <p>Code below:</p>
  
  <pre><code>quartz(width = 8,height = 3.5);
  
  plot(DATE,tag$Depth, type='l', ylim=c(ymax,ymin), xlab="Date",ylab="Depth (m)");
  
  axis(side=3, labels=DATE, at=DATE);
  # works but I need to only show one date/tick every week
  </code></pre>
  
  <p>I know I need to specify something different for the at= expression, but since it needs to be the same as the labels= expression I can't seem to work out the syntax. I tried defining the set of dates to be displayed as follows, but without success so far...</p>
  
  <pre><code>newDATE &lt;- seq(as.Date(DATE[1], format = "%d/%m/%Y"), tail(as.Date(DATE), 1), by = "week") 
  </code></pre>
  
  <p>Suggestions welcome! Thanks in advance for your help!</p>