title: ggplot removes all my data when I set x and y limits
body: <p>I'm using R to generate a plot from a table. </p>
  
  <p>I used the command <code>a &lt;- read.table("table.txt")</code> and that worked fine. When I type "a" it prints out my full table correctly.</p>
  
  <p>I also entered <code>library(ggplot2)</code> so I could use ggplot.</p>
  
  <p>Here was my first command:</p>
  
  <pre><code>ggplot(a, aes(x=V2, y=V5, group=V7)) + 
     geom_point(size=4, aes(col=V7)) + xlab("Rank") + 
     ylab("Inter-helix angle (Degree)") + 
     opts(legend.position="none") 
  </code></pre>
  
  <p>this command generated all the points, but when I added <code>xlim(0,110) + ylim(0,110)</code> to the end, it gave me a warning and said "Removed 101 Rows containing missing values" (my table is 101 rows long). The plot that was generated was completely empty. What happened?</p>