title: Persistent invalid graphics state error when using ggplot2
body: <p>I believe my dataframe is okay and my code is okay.  In fact, I have eliminated parts of the dataframe and most of the graphing code to make things as basic as possible.  But still, I get:</p>
  
  <pre><code>Error in .Call.graphics(C_palette2, .Call(C_palette2, NULL)) : 
    invalid graphics state
  </code></pre>
  
  <p>What is wrong here?  Here is the data: </p>
  
  <pre><code> date   trt var val
  1/8/2008    cc  sw5 0.2684138
  1/8/2008    cc  sw15    0.2897586
  1/8/2008    cc  sw5 0.2822414
  2/8/2008    cc  sw5 0.2494583
  2/8/2008    cc  sw5 0.2692917
  2/8/2008    cc  sw15    0.2619167
  2/8/2008    cc  sw5 0.204375
  3/8/2008    cc  sw5 0.2430625
  3/8/2008    cc  sw5 0.2654375
  3/8/2008    cc  sw5 0.2509583
  3/8/2008    cc  sw5 0.2055625
  1/8/2008    ccw sw15    0.2212414
  1/8/2008    ccw sw5 0.3613448
  1/8/2008    ccw sw5 0.2607586
  2/8/2008    ccw sw5 0.2087917
  2/8/2008    ccw sw15    0.3390417
  2/8/2008    ccw sw5 0.2436458
  2/8/2008    ccw sw5 0.290875
  3/8/2008    ccw sw5 0.20175
  3/8/2008    ccw sw15    0.328875
  3/8/2008    ccw sw5 0.2328958
  3/8/2008    ccw sw5 0.2868958
  </code></pre>
  
  <p>When I work with this data, I specify dates like this:</p>
  
  <pre><code>df&lt;-df[order(as.Date(df$date,format="%d/%m/%Y")),,drop=FALSE]
  </code></pre>
  
  <p>and here I want to make a scatterplot:</p>
  
  <pre><code>ggplot(data = df,aes(x = date,y = val)) + 
   geom_point(aes(group = trt))
  </code></pre>