title: Error from ggplot2 plotting date data -- missing value where TRUE/FALSE needed
body: <p>I'm trying to plot some grades I'm pulling from an external source.  The date format comes in looking like this:</p>
  
  <pre><code>2011-08-23T17:07:05
  </code></pre>
  
  <p>So I parse it with <code>strptime(date, "%FT%X")</code> and get a <code>POSIXlt</code>.  I end up with a complete data frame that looks like this:</p>
  
  <pre><code>                  date    subject  grade
  1  2011-08-23 17:07:05 AP Biology  95.83
  2  2011-08-24 17:07:03 AP Biology  95.83
  3  2011-08-25 17:08:27 AP Biology  95.83
  4  2011-08-17 17:05:54 US History 157.14
  5  2011-08-18 17:05:24 US History 157.14
  6  2011-08-19 17:05:35 US History 157.14
  7  2011-08-22 17:06:25 US History 157.14
  8  2011-08-23 17:07:05 US History 157.14
  9  2011-08-24 17:07:03 US History 157.14
  10 2011-08-25 17:08:27 US History 157.14
  11 2011-08-19 17:05:35   Yearbook   0.00
  12 2011-08-22 17:06:25   Yearbook   0.00
  13 2011-08-23 17:07:05   Yearbook 100.00
  14 2011-08-24 17:07:03   Yearbook 100.00
  15 2011-08-25 17:08:27   Yearbook 100.00
  </code></pre>
  
  <p>With the following structure:</p>
  
  <pre><code>'data.frame':   15 obs. of  3 variables:
   $ date   : POSIXlt, format: "2011-08-23 17:07:05" "2011-08-24 17:07:03" ...
   $ subject: Factor w/ 3 levels "AP Biology","US History",..: 1 1 1 2 2 2 2 ...
   $ grade  : num  95.8 95.8 95.8 157.1 157.1 ...
  </code></pre>
  
  <p>When I try to plot this data:</p>
  
  <pre><code>&gt; ggplot(data=grades, aes(date, grade, factor=subject)) + geom_line()
  Error in if (length(range) == 1 || diff(range) == 0) { : 
    missing value where TRUE/FALSE needed
  </code></pre>
  
  <p>I don't know what I'm doing wrong here.  I narrowed it down to the date handling by doing this:</p>
  
  <pre><code>ggplot(data=grades,
         aes(seq(length(grades[,1])),
             grade, color=subject)) + geom_line()
  </code></pre>
  
  <p>... but how do I do the date handling correctly?</p>