title: Plot same x-axis for two different data sets
body: <p>I have two separated data sets: <code>AUD-USD</code> and <code>CHF-JPY</code> and they look like this (snapshots every 100 milliseconds):</p>
  
  <pre><code>currency,  price, datetime
  
  CHF/JPY,   93.84, 2011-09-06 08:00:00.000   #from 8:00 to 8:30
  
  AUD-USD,   1.84,  2011-09-06 07:00:00.000   #from 7:00 to 9:00
  
  mydata$datetime &lt;- as.POSIXct(data$datetime, tz="GMT")
  
  time1&lt;-as.POSIXct("2011-09-06 7:00:00", tz="GMT")
  time2&lt;-as.POSIXct("2011-09-06 9:00:00", tz="GMT")
  plot(price~datetime, xaxt="n",main="", xlab="Time", ylab="Price",  data=mydata)
  </code></pre>
  
  <p>I tried this:</p>
  
  <pre><code>axis(1,at=seq(time1,time2,by="hour"),label=seq(time1,time2,by="hour"))
  </code></pre>
  
  <p>I want to have <em>two separated graphs</em> with the same x-axis for both currency pairs to compare them. </p>
  
  <pre><code>07:00, 07:30, 08:00, 08:30, 09:00
  </code></pre>
  
  <p>Since the <code>CHF-JPY</code> data is only from 8:00 to 8:30, I end up with different x-axis. </p>
  
  <p>Sorry, I do not have enough reputation to post images.</p>
  
  <hr>
  
  <p>EDIT: </p>
  
  <ul>
  <li><p><code>dput(mydata)</code></p>
  
  <pre><code>structure(list(currency = c("CHF/JPY"), price = c(93.84), Volume = c(1), datetime = structure(c(1315296191.6))))
  </code></pre></li>
  </ul>