title: setting xlim and breaks on x axis in plot function
body: <p>I have a df like this:</p>
  
  <pre><code>Value   Time
  10      2012-06-23 01:00:00
  20      2012-06-23 02:00:00
  </code></pre>
  
  <p>time is formated as <code>as.PoSIXct(df$Time, format="%Y-%m-%d %H:%M:%S")</code></p>
  
  <p>I am trying to set the breaks in axis to show xaxis tick mark every 2 hours as this:</p>
  
  <pre><code>plot(df$Time, df$Value, ylim(0,100), xlim(seq(c("2012-06-23 01:00:00", "2012-06-23 23:00:00", 2)))
  </code></pre>
  
  <p>I get invalid xlim value. any ideas?</p>
  
  <p>I have tried this line, still not working:</p>
  
  <pre><code>plot(df$Time, df$Value, col="red", type="l", xlab="Date", ylab="% Memory Utilization", xlim=seq(c(as.POSIXct("2012-06-23 01:00:00"), as.POSIXct("2012-06-23 23:00:00"), by="hour")))
  </code></pre>