title: ggplot2 uses incorrect x-values for time
body: <p>I'm plotting data from 18:00 until 21:00. As you can see in the first screenshot, the data has a large time-gap from 18:13-20:47, but this is not shown in the chart. So the dots are wrongly plotted. During that time, also a gap in the y-values arised. That is actually visible in the chart.</p>
  
  <p><a href="https://i.stack.imgur.com/IYqEq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/IYqEq.png" alt="enter image description here"></a></p>
  
  <p>The second screenshot shows the plot. There's a gap in the y values, but the gap in the x values is missing.</p>
  
  <pre><code>usd = dbSendQuery(mydb, "SELECT timestamp, refPrice FROM `balance` where timestamp&gt;'2017-09-27 18:00:00' AND timestamp &lt; '2017-09-27 21:00:00' ORDER BY `id` DESC")
  data = fetch(usd, n=-1)
  ggplot(aes(x = timestamp, y = refPrice), data=data) + geom_point()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/NOYlf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NOYlf.png" alt="enter image description here"></a></p>
  
  <p>mysql column:</p>
  
  <pre><code>`timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
  </code></pre>
  
  <p>It's also a pity that all timestamps are shown on the x-axis, so that they overlap.</p>